Code generation


This section is a work in progress, documenting the most recent code generation framework, which is in the package brian.experimental.codegen2.


To generate code, we start with a basic statement or set of statements we want to evaluate for all neurons, or for all synapses, and then apply various transformations to generate code that will do this. We start from a structured, language-invariant representation of the set of basic statements. We then ‘resolve’ the unknown symbols in it. This is done recursively, the resolution of each symbol can add vectorised statements or loops to the current representation, and add data to a namespace that will be associated to the final code. Symbols will be things like a NeuronGroup state variable, or a synaptic weight value. The output of this process is a new, more complicated structured representation, including things like loops if necessary. Next, we convert this structured representation into a code string. Finally, this code string is JIT-compiled into an executable object.

Using numerical integration generation

You can use Brian’s equations format to generate C/C++ code for a numerical integration step, for example:

eqs = '''
dv/dt = (ge+gi-(v+49*mV))/(20*ms) : volt
dge/dt = -ge/(5*ms) : volt
dgi/dt = -gi/(10*ms) : volt
code, vars, params = make_c_integrator(eqs, method=euler, dt=0.1*ms)
print code

has output:

double _temp_v = 50.0*ge + 50.0*gi - 50.0*v - 2.45;
double _temp_ge = -200.0*ge;
double _temp_gi = -100.0*gi;
v += _temp_v*0.0001;
ge += _temp_ge*0.0001;
gi += _temp_gi*0.0001;

See the documentation for the function make_c_integrator().

Using the code generation package

The basic way to use the code generation module is as follows:

  1. Create a Block of Statement objects which you want to execute. You can use statements_from_codestring() to do this.
  2. Create a dictionary of Symbol objects corresponding to the symbols in the block above.
  3. Call CodeItem.generate() with the specified language and symbols, to give you a Code object.
  4. Optionally, insert additional data into the namespace of the Code object.
  5. Use the Code object via code(name1=val1, name2=val2) where the name=val are to be inserted into the namespace before the code is called.

This process is very clearly illustrated in the source code for CodeGenStateUpdater.

Structure of the package

The following are the main elements of the code generation package:

This is the output of the code generation package, a compilable/compiled code fragment, along with a namespace in which it is executed.
Used to specify which language the output should have.
Before code is converted into a specific language, it is stored in a language-invariant format consisting of CodeItem objects, which can in turn contain other CodeItem objects. The main derived classes from this are Block and Statement. The first can contain a series of statements, or it can be a for loop, an if block, etc. A Statement can be a MathematicalStatement or CodeStatement. The former is for things like x=y*z and the latter for things like x = arr[index].
Symbol, resolve()
A CodeItem with unresolved dependencies needs to be resolved by the function resolve(). Each unresolved depdendency should correspond to a Symbol which knows how to modify a CodeItem in order to resolve itself. For example, a NeuronGroupStateVariableSymbol will insert the NeuronGroup state variable value into the namespace, create a new array symbol like __arr_V for symbol V, and resolve itself either by doing nothing (in Python, as the variable is already vectorised), or by introducing a loop (in C++), or by setting the index variable as the kernel thread (for GPU). For more details, see the section on resolution below.
make_integration_step(), euler(), rk2(), exp_euler()
Numerical integration schemes, each integration scheme (such as euler()) converts a set of differential equations into a sequence of MathematicalStatement objects comprising an integration step.
CodeGenStateUpdater, CodeGenThreshold, CodeGenReset, CodeGenConnection
Brian objects using code generation.

Resolution process


We start with a worked example. Consider the statement:

V = V+1

Here V is a NeuronGroup state variable. We wish to transform this into code that can be executed. In the case of Python, the output would look like:

_neuron_index = slice(None)
V = _arr_V[_neuron_index]
_arr_V[_neuron_index] = V+1

The symbol _arr_V would be added directly to the namespace.

In the case of C++ it would look like:

for(int _neuron_index=0; _neuron_index<_len__arr_V; _neuron_index++)
        double &V = _arr_V[_neuron_index];
        V = V+1;

Here the symbols _arr_V and _len__arr_V` would be added to the namespace. The reason for these complicated names is to do with making the code as generic as possible, not introducing namespace clashes (symbols starting with _ are reserved), etc.

The way the process works is we start with the statement V=V+1 and a Symbol object with name V, specifically a NeuronGroupStateVariableSymbol. The statement V=V+1 depends on V with both a Read and Write dependency. We therefore have to ‘resolve’ the symbol V. To do this we call the method resolve() on V.

In the case of Python, this gives us:

V = _arr_V[_neuron_index]
_arr_V[_neuron_index] = V+1

It adds _arr_V to the namespace, and creates a dependency on _neuron_index. The reason that V=V+1 is translated to _arr_V[_neuron_index] = V+1 is that on the left hand side we have a write variable, and on the right hand side we have a read variable. In Python, when vectorising, we have no choice but to give the underlying array with its slice when writing to an array. However, at this point the code generation framework doesn’t know what _neuron_index will be, so it could be, for example, an array of indices. In this case, suppose we did V*V it would be more efficient to compute V=_arr_V[_neuron_index] and then compute V*V than to compute _arr_V[_neuron_index]*_arr_V[_neuron_index], and in the case where _neuron_index=slice(None) it is no slower, so we always do this.

In the case of C++, the first resolution step gives us:

double &V = _arr_V[_neuron_index];
V = V+1;

For the second resolution step, we need to resolve _neuron_index, which is a symbol of type SliceIndex, telling us that _neuron_index varies over all neurons. Note that we could also have _neuron_index being an ArrayIndex, for examples spikes, and then this could be used for a reset operation (we would iterate only over those indices of neurons which had spiked). Here though, we iterate over all neurons. In Python, calling the resolve() method of _neuron_index gives us:

_neuron_index = slice(None)
V = _arr_V[_neuron_index]
_arr_V[_neuron_index] = V+1

and in C++:

for(int _neuron_index=0; _neuron_index<_len__arr_V; _neuron_index++)
        double &V = _arr_V[_neuron_index];
        V = V+1;

In both cases, the _neuron_index symbol is resolved and the process is complete.

Note that we have actually mixed two stages here, the stage of generating a structured representation of the code using CodeItem objects, and the stage of generating code strings using CodeItem.convert_to(). In fact, the converting of, for example, V to _arr_V[_neuron_index] only happens at the second stage.


The first stage, acting on the structured representation of nested CodeItem objects is resolved using the function resolve(). This calls Symbol.resolve() for each of the symbols in turn. The resolution order is determined by an optimal efficiency algorithm, see the reference documentation for resolve() for the full algorithm description.

Symbol.resolve() can do an arbitrary transformation of the input CodeItem, but typically it will either do something like:


Or something like:

for name in array:

See the reference documentation for Symbol.resolve(), and the documentation for the most important symbols:


This step is relatively straightforward, each CodeItem object has its convert_to method called iteratively. The important one is in MathematicalStatement, where the left hand side usage is replaced by Symbol.write() and the right hand side usage is replaced by In addition, at this stage the syntax of mathematical statements is corrected, e.g. Python’s x**y is replaced by C++’s pow(x,y) using sympy.

Code generation in Brian

The four objects used for code generation in Brian are:

Used for numerical integration, see above and reference documentation.
Used for computing a threshold function.
Used for computing post-spike reset.
Used for synaptic propagation.

Numerical integration

An integration scheme is generated from an Equations object using the make_integration_step() function. See reference documentation for that function for details.

This is carried out by CodeGenStateUpdater, which can be used as a Brian brian.StateUpdater object in brian.NeuronGroup.

As an example, for Euler integration, the differential equations:

dx/dt = expr

are separated by Equations into variable x with expression expr. This then becomes:

_temp_x := expr
x += _temp_x*dt

This can then be resolved by the code generation mechanisms described already.

Synaptic propagation

TODO: synaptic propagation, including docstrings and code comments

NOTE: GPU functionality not included for synaptic propagation yet.



GPU code is highly transitional, many details may change in the future.

GPU code is handled by five classes:

GPULanguage (derived from CLanguage)
Identifies the language as CUDA, and stores a singleton GPUManager object which is used to manage the GPU.
GPUCode (derived from Code)
Returned from the code generation process, but mostly just acts as a proxy to GPUManager.
Handles the final stage of taking a partially generated kernel (without the vectorisation over threads) and computing the final kernel (using vectorisation over threads). Also adds data to the GPUSymbolMemoryManager.
Manages the GPU generally. Stores a set of kernels (GPUKernel) and manages memory via GPUSymbolMemoryManager. Handles joining the memory management code and kernel code into a single source file, and compiling it.
Handles allocation of GPU memory for symbols.

For more details, see the reference documentation for the classes in the order above.

Note that CodeGenConnection is the only code generation version of a Brian class which is not GPU enabled at present.

Extending code generation

To extend code generation, you will probably need to add new Symbol classes. Read the documentation for this class to start, and the documentation for the most important symbols:

See also CodeItem, particularly the process described in CodeItem.generate().

Inheritance diagrams

The overall structure of the classes in the code generation package are included below, for reference.


Inheritance diagram of brian.experimental.codegen2.languages, GPULanguage

Code objects

Inheritance diagram of brian.experimental.codegen2.codeobject, GPUCode

Code items

Inheritance diagram of brian.experimental.codegen2.codeitems, brian.experimental.codegen2.statements, brian.experimental.codegen2.blocks


Inheritance diagram of brian.experimental.codegen2.equations, brian.experimental.codegen2.expressions


Inheritance diagram of brian.experimental.codegen2.symbols

Resolution and code output

Inheritance diagram of brian.experimental.codegen2.dependencies, brian.experimental.codegen2.formatting


Inheritance diagram of brian.experimental.codegen2.integration


Inheritance diagram of

Brian objects


Inheritance diagram of brian.experimental.codegen2.connection


Inheritance diagram of brian.experimental.codegen2.reset

State updater

Inheritance diagram of brian.experimental.codegen2.stateupdater


Inheritance diagram of brian.experimental.codegen2.threshold



class brian.experimental.codegen2.Block(*args)

Contains a list of CodeItem objects which are considered to be executed in serial order. The list is passed as arguments to the init method, so if you want to pass a list you can initialise as:

block = Block(*items)
class brian.experimental.codegen2.ControlBlock(start, end, contents, dependencies, resolved)

Helper class used as the base for various control structures such as for loops, if statements. These are typically not language-invariant and should only be output in the resolution process by symbols (which know the language they are resolving to). Consists of strings start and end, a list of contents (as for Block), and explicit sets of dependencies and resolved (these are self-dependencies/resolved). The output code consists of the start string, the indented converted contents, and then the end string. For example, for a C for loop, we would have start='for(...){ and end='}'.

convert_to(language, symbols={}, namespace={})
class brian.experimental.codegen2.ForBlock(start, end, contents, dependencies, resolved)

Simply a base class, does nothing.

class brian.experimental.codegen2.PythonForBlock(var, container, content, dependencies=None, resolved=None)

A for loop in Python, the structure is:

for var in container:

Where var and container are strings, and content is a CodeItem or list of items.

Dependencies can be given explicitly, or by default they are Read(x) for each word x in container. Resolved can be given explicitly, or by default it is set(var).

class brian.experimental.codegen2.CForBlock(var, spec, content, dependencies=None, resolved=None)

A for loop in C, the structure is:


You specify a string var which is the variable the loop is iterating over, and a string spec should be of the form 'int i=0; i<n; i++'. The content is a CodeItem or list of items. The dependencies and resolved sets can be given explicitly, or by default they are extracted, respectively, from the set of words in spec, and set([var]).

class brian.experimental.codegen2.IfBlock(start, end, contents, dependencies, resolved)

Just a base class.

class brian.experimental.codegen2.PythonIfBlock(cond, content, dependencies=None, resolved=None)

If statement in Python, structure is:

if cond:

Dependencies can be specified explicitly, or are automatically extracted as the words in string cond, and resolved can be specified explicitly or by default is set().

class brian.experimental.codegen2.CIfBlock(cond, content, dependencies=None, resolved=None)

If statement in C, structure is:


Dependencies can be specified explicitly, or are automatically extracted as the words in string cond, and resolved can be specified explicitly or by default is set().


class brian.experimental.codegen2.CodeItem

An item of code, can be anything from a single statement corresponding to a single line of code, right up to a block with nested loops, etc.

Should define the following attributes (default values are provided):

The set of dependencies which have been resolved in this item, including in items contained within this item. Default value: the union of selfresolved and subresolved. Elements of the set should be of type Dependency (i.e. Read or Write).
The set of dependencies resolved only in this item, and not in subitems. Default value: set().
The set of dependencies resolved in subitems, default value is the union of item.dependencies for each item in this item. Requires the CodeItem to have an iterator, i.e. a method __iter__.
dependencies, selfdependencies, subdependencies
As above for resolved, but giving the set of dependencies in this code. The default value for dependencies takes the union of selfdependencies and subdependencies and removes all the symbols in resolved.

This structure of having default implementations allows several routes to derive a class from here, e.g.:

Simply defines a list attribute contents which is a sequence of items, and implements __iter__ to return iter(contents).
Defines a fixed string which is not language-invariant, and a fixed set of dependencies and resolved. The convert_to() method simply returns the fixed string. Does not define an __iter__ method because the default values for dependencies and resolved are overwritten.
convert_to(language, symbols={}, namespace={})

Returns a string representation of the code for this item in the given language. From the user point of view, you should call generate(), but in developing new CodeItem derived classes you need to implement this. The default behaviour is simply to concatenate the strings returned by the subitems.

generate(name, language, symbols, namespace=None)

Returns a Code object. The method resolves the symbols using resolve(), converts to a string with convert_to() and then converts that to a Code object with Language.code_object().



class brian.experimental.codegen2.Code(name, code_str, namespace, pre_code=None, post_code=None, language=None)

The basic Code object used for all Python/C/GPU code generation.

The Code object has the following attributes:

The name of the code, should be unique. This matters particularly for GPU code which uses the name attribute for the kernel function names.
A representation of the code in string form
A dictionary of name/value pairs in which the code will be executed
An optional value (can be None) consisting of some representation of the compiled form of the code
pre_code, post_code
Two optional Code objects which can be in the same or different languages, and can share partially or wholly the namespace. They are called (respectively) before or after the current code object is executed.
A Language object that stores some global settings and state for all code in that language.

Each language (e.g. PythonCode) extends some or all of the methods:

Unsurprisingly used for initialising the object, should call Code.__init__ with all of its arguments.
Compiles the code, if necessary. If not necessary, set the code_compiled value to any dummy value other than None.
Runs the compiled code in the namespace.

It will usually not be necessary to override the call mechanism:

Calls pre_code(**kwds), updates the namespace with kwds, executes the code (calls and then calls post_code(**kwds).
class brian.experimental.codegen2.PythonCode(name, code_str, namespace, pre_code=None, post_code=None, language=None)
class brian.experimental.codegen2.CCode(name, code_str, namespace, pre_code=None, post_code=None, language=None)


class brian.experimental.codegen2.CodeGenConnection(*args, **kwds)
class brian.experimental.codegen2.DenseMatrixSymbols
class SynapseIndex(M, name, weightname, language, sourceindex='_source_index', targetlen='_target_len')
resolve(read, write, vectorisable, item, namespace)
class TargetIndex(M, name, weightname, language, index='_synapse_index', targetlen='_target_len')
load(read, write, vectorisable)
supported_languages = ['python', 'c']
class Value(M, name, language, index='_synapse_index')
class brian.experimental.codegen2.SparseMatrixSymbols
class SynapseIndex(M, name, weightname, language, sourceindex='_source_index')
resolve(read, write, vectorisable, item, namespace)
class TargetIndex(M, name, weightname, language, index='_synapse_index')
class Value(M, name, language, index='_synapse_index')


class brian.experimental.codegen2.Dependency(name)

Base class for Read and Write dependencies.

A dependency marks that a CodeItem depends on a given symbol. Each dependency has a name.

class brian.experimental.codegen2.Read(name)

Used to indicate a read dependency, i.e. the value of the symbol is read.

class brian.experimental.codegen2.Write(name)

Used to indicate a write dependency, i.e. the value of the symbol is written to.


Returns the set of names of the variables which are either read to or written to in a set of dependencies.


brian.experimental.codegen2.freeze_with_equations(inputcode, eqs, ns)

Returns a frozen version of inputcode with equations and namespace.

Replaces each occurrence in inputcode of a variable name in the namespace ns with its value if it is of int or float type. Variables with names in brian.Equations eqs are not replaced, and neither are dt or t.


Returns a frozen set of equations.

Each expression defining an equation is frozen as in freeze_with_equations().


class brian.experimental.codegen2.Expression(expr)

A mathematical expression such as x*y+z.

Has an attribute dependencies which is Read(var) for all words var in expr.

Has a method convert_to() defined the same way as CodeItem.convert_to().

convert_to(language, symbols={}, namespace={})

Converts expression into a string for the given language using the given set of symbols. Replaces each Symbol appearing in the expression with, and if the language is C++ or GPU then uses sympy.CCodePrinter().doprint() to convert the syntax, e.g. x**y becomes pow(x,y).


brian.experimental.codegen2.word_substitute(expr, substitutions)

Applies a dict of word substitutions.

The dict substitutions consists of pairs (word, rep) where each word word appearing in expr is replaced by rep. Here a ‘word’ means anything matching the regexp \bword\b.

brian.experimental.codegen2.flattened_docstring(docstr, numtabs=0, spacespertab=4, split=False)

Returns a docstring with the indentation removed according to the Python standard

split=True returns the output as a list of lines

Changing numtabs adds a custom indentation afterwards

brian.experimental.codegen2.indent_string(s, numtabs=1, spacespertab=4, split=False)

Indents a given string or list of lines

split=True returns the output as a list of lines


Return all the identifiers in a given string expr, that is everything that matches a programming language variable like expression, which is here implemented as the regexp \b[A-Za-z_][A-Za-z0-9_]*\b.


Removes all empty lines from the multi-line string s.



GPU code is highly transitional, many details may change in the future.

class brian.experimental.codegen2.GPUKernel(name, code, namespace, mem_man, maxblocksize=512, scalar='double', force_sync=True)

Generates final kernel source code and used to launch kernels.

Used in conjunction with GPUManager. Each kernel is prepared with prepare() which generates source code and adds symbols to the GPUSymbolMemoryManager. The GPUManager compiles the code and sets the gpu_func attribute, and the kernel can then be called via run().

The initialisation method extracts variable _gpu_vector_index from the namespace and stores it as attribute index, and _gpu_vector_slice as the pair (start, end).


Generates kernel source code and adds symbols to memory manager.

We extract the number of GPU indices from the namespace, _num_gpu_indices.

We loop through the namespace, and for each value determine it to be either an array or a single value. If it is an array, then we place it in the GPUSymbolMemoryManager, otherwise we add it to the list of arguments provided to the function call. This allows scalar variables like t to be transmitted to the kernel function in its arguments.

We then generate a kernel of the following (Python template) form:

__global__ void {name}({funcargs})
    int {vector_index} = blockIdx.x * blockDim.x + threadIdx.x;

We also compute the block size and grid size using the user provided maximum block size.


Calls the pycuda GPU function prepare() method for low-overhead function calls.


Calls the function on the GPU, extracting the scalar variables in the argument list from the namespace.

class brian.experimental.codegen2.GPUManager(force_sync=True, usefloat=False)

This object controls everything on the GPU.

It uses a GPUKernel object for managing kernels, and a GPUSymbolMemoryManager object for managing symbol memory.

The class is used by:

  1. Adding several kernels using add_kernel()
  2. Calling prepare() (see method documentation for details)
  3. Run code with run()

Memory is mirrored on GPU and CPU. In the present implementation, in the development phase only, each call to run() will copy all symbols from CPU to GPU before running the GPU kernel, and afterwards copy all symbols from GPU back to CPU. In the future, this will be disabled and symbol memory copies will be handled explicitly by calls to methods copy_to_device() and copy_to_host().

add_kernel(name, code, namespace)

Adds a kernel with the given name, code and namespace. Creates a GPUKernel object.


Proxy to GPUSymbolMemoryManager.add_symbols().


Compiles code using pycuda.compiler.SourceModule and extracts kernel functions with pycuda.compiler.SourceModule.get_function(). The GPUKernel.gpu_func attribute is set for each kernel.


Proxy to GPUSymbolMemoryManager.copy_to_device().


Proxy to GPUSymbolMemoryManager.copy_to_host().


Combines kernel source into one source file, and adds memory management kernel functions. These simple kernels simply copy a pointer to a previously specified name. This is necessary because when pycuda is used to allocate memory, it doesn’t give it a name only a pointer, and the kernel functions use a named array.

Calls GPUSymbolMemoryManager.generate_code().


Copies allocated memory pointers to named global memory pointer variables so that kernels can use them. The kernel names to do this are in the GPUSymbolMemoryManager.symbol_upload_funcnames dict (keys are symbol names), and the allocated pointers are in the GPUSymbolMemoryManager.device dict.


Not used at present. Will be used to combine multiple kernels with the same vectorisation index for efficiency.


Compiles code and initialises memory.

Performs the following steps:

  1. GPUKernel.prepare() is called for each kernel, converting the partial code into a complete kernel, and adds symbols to the GPUSymbolMemoryManager, which allocates space on the GPU and copies data to it from the CPU.
  2. generate_code() is called, combining individual kernels into one source file, and adding memory management kernels and declarations.
  3. compile() is called, which JIT-compiles the code using pycuda.
  4. initialise_memory() is called, which allocates memory

Runs the named kernel. Calls Note that all symbols are copied to and from the GPU before and after the kernel run, although this is only for the development phase and will change later.

class brian.experimental.codegen2.GPUSymbolMemoryManager(usefloat=False)

Manages symbol memory on the GPU.

Stores an attribute device and host which are dicts, with keys the symbol names, and values pycuda.gpuarray.GPUArray and numpy.ndarray respectively. Add symbols with add_symbols(), which will allocate memory.


Adds a collection of symbols.

Each item in items is of the form (symname, hostarr, devname) where symname is the symbol name, hostarr is the numpy.ndarray containing the data, and devname is the name the array pointer should have on the device.

Allocates memory on the device, and copies data to the GPU.


Copy the memory in the numpy.ndarray for symname to the allocated device memory. If symname==True, do this for all symbols. You can also pass a list for symname.


As for copy_to_device() but copies memory from device to host.


Generates declarations for array pointer names on the device, and kernels to copy device pointers to the array pointers. General form is:

__device__ {dtypestr} *{name};
__global__ void set_array_{name}({dtypestr} *_{name})
    {name} = _{name};

Stores the kernel function names in attribute symbol_upload_funcnames (dict with keys being symbol names).

Returns a string with declarations and kernels combined.


The list of symbol names managed.

class brian.experimental.codegen2.GPUCode(name, code_str, namespace, pre_code=None, post_code=None, language=None)

Code object for GPU.

For the user, works as the same as any other Code object. Behind the scenes, source code is passed to the GPUManager gpu_man from the GPULanguage object, via GPUManager.add_kernel(). Compilation is handled by GPUManager.prepare(), and running code by


Simply calls GPUManager.prepare().


Simply runs the kernel via

class brian.experimental.codegen2.GPULanguage(scalar='double', gpu_man=None, force_sync=True)

Language object for GPU.

Has an attribute gpu_man, the GPUManager object responsible for allocating, copying memory, etc. One is created if you do not specify one.


alias of GPUCode


class brian.experimental.codegen2.EquationsContainer(eqs)

Utility class for defining numerical integration scheme

Initialise with a set of equations eqs. You can now iterate over this object in two ways, firstly over all the differential equations:

for var, expr in eqscontainer:
    yield f(expr)

Or over just the differential equations with nonzero expressions (i.e. not including dx/dt=0 for parameters):

for var, expr in eqscontainer.nonzero:
    yield f(expr)

Here var is the name of the symbol, and expr is a string, the right hand side of the differential equation dvar/dt=expr.

Also has attributes:

The symbol names for all the differential equations
The symbol names for all the nonzero differential equations
brian.experimental.codegen2.make_integration_step(method, eqs)

Return an integration step from a method and a set of equations.

The method should be a function method(eqs) which receives a EquationsContainer object as its argument, and yield s statements. For example, the euler() integration step is defined as:

def euler(eqs):
    for var, expr in eqs.nonzero:
        yield '_temp_{var} := {expr}'.format(var=var, expr=expr)
    for var, expr in eqs.nonzero:
        yield '{var} += _temp_{var}*dt'.format(var=var, expr=expr)

Euler integration


2nd order Runge-Kutta integration


Exponential-Euler integration


class brian.experimental.codegen2.Language(name)

Base class for languages, each should provide a name attribute, and a method code_object().

code_object(name, code_str, namespace)

Return a Code object from a given name, code string code_str and namespace. If the class has a class attribute CodeObjectClass, the default implementation returns:

CodeObjectClass(name, code_str, namespace, language=self)
class brian.experimental.codegen2.PythonLanguage

Python language.


alias of PythonCode

class brian.experimental.codegen2.CLanguage(scalar='double')

C language.

Has an attribute scalar='double' which gives the default type of scalar values (used when dtype is not specified). This can be used, for example, on the GPU where double may not be available.


alias of CCode


brian.experimental.codegen2.make_c_integrator(eqs, method, dt, values=None, scalar='double', timeunit=second, timename='t')

Gives C/C++ format code for the integration step of a differential equation.

The equations, can be an brian.Equations object or a multiline string in Brian equations format.
The integration method, typically euler(), rk2() or exp_euler(), although you can pass your own integration method, see make_integration_step() for details.
The value of the timestep dt (in Brian units, e.g. 0.1*ms)
Optional, dictionary of mappings variable->value, these values will be inserted into the generated code.
By default it is 'double' but if you want to use float as your scalar type, set this to 'float'.
The name of the time variable (if used). In Brian this is ‘t’, but you can change it to ‘T’ or ‘time’ or whatever. This can be used if you want users to specify time in Brian form (‘t’) but the context in which this code will be used (e.g. another simulator) specifies time with a different variable name (e.g. ‘T’).
The unit of the time variable, scaled because Brian expects time to be in seconds.

Returns a triple (code, vars, params):

The C/C++ code to perform the update step (string).
A list of variable names.
A list of per-neuron parameter names.


class brian.experimental.codegen2.CodeGenReset(group, inputcode, language, level=0)


brian.experimental.codegen2.resolve(item, symbols, namespace=None)

Resolves symbols in item in the optimal order.

The first stage of this algorithm is to construct a dependency graph on the symbols.

The optimal order is resolve loops as late as possible. We actually construct the inverse of the resolution order, which is the intuitive order (i.e. if the first thing we do is loop over a variable, then that variable is the last symbol we resolve).

We start by finding the set of symbols which have no dependencies. The graph is acyclic so this always possible. Then, among those candidates, if possible we choose loopless symbols first (this corresponds to executing loops as late as possible). With this symbol removed from the graph we repeat until all symbols are placed in order.

We then resolve in reverse order (because we start with the inner loop and add code outwards). At the beginning of this stage, vectorisable is set to True. But after we encounter the first multi-valued symbol we set vectorisable to False (we can only vectorise one loop, and it has to be the innermost one). This vectorisation is used by both Python and GPU but not C++. Each resolution step calls CodeItem.resolve() on the output of the previous stage.


class brian.experimental.codegen2.Statement

Just a base class, supposed to indicate single line statements.

class brian.experimental.codegen2.CodeStatement(code, dependencies, resolved)

A language-specific single line of code, which should only be used in the resolution step by a Symbol which knows the language it is resolving to. The string code and the set of dependencies and resolved have to be given explicitly.

convert_to(language, symbols={}, namespace={})
class brian.experimental.codegen2.CDefineFromArray(var, arr, index, dependencies=None, resolved=None, dtype=None, reference=True, const=False)

Define a variable from an array and an index in C.

For example:

double &V = __arr_V[neuron_index];

Initialisation arguments are:

The variable being defined, a string.
A string representing the array.
A string giving the index.
Given explicitly, or by default use set([Read(arr), Read(index)]).
Given explicitly, or by default use set([var]).
The numpy data type of the variable being defined.
Whether the variable should be treated as a C++ reference (e.g. double &V = ... rather than double V = .... If the variable is being written to as well as read from, use reference=True.
Whether the variable can be defined as const, specify this if only reading the value and not writing to it.
class brian.experimental.codegen2.MathematicalStatement(var, op, expr, dtype=None)

A single line mathematical statement.

The structure is var op expr.

The left hand side of the statement, the value being written to, a string.
The operation, can be any of the standard Python operators (including += etc.) or a special operator := which means you are defining a new symbol (whereas = means you are setting the value of an existing symbol).
A string or an Expression object, giving the right hand side of the statement.
If you are defining a new variable, you need to specify its numpy dtype.

If op==':=' then this statement will resolve var, otherwise it will add a Write dependency for var. The other dependencies come from expr.

convert_to(language, symbols={}, tabs=0, namespace={})

When converting to a code string, the following takes place:

  • If the LHS variable is in the set of symbols, then the LHS is replaced by sym.write()
  • The expression is converted with Expression.convert_to().
  • If the operation is definition op==':=' then the output is language dependent. For Python it is lhs = rhs and for C or GPU it is dtype lhs = rhs.
  • If the operation is not definition, the statement is converted to lhs op rhs.
  • If the language is C/GPU the statement has ; appended.
brian.experimental.codegen2.statements_from_codestring(code, eqs=None, defined=None, infer_definitions=False)

Generate a list of statements from a user-defined string.

The input code string, a multi-line string which should be flat, no indents.
A Brian Equations object, which is used to specify a set of already defined variable names if you are using infer_definitions.
A set of symbol names which are already defined, if you are using infer_definitions.
Set to True to guess when a line of the form a=b should be inferred to be of type a:=b, as user-specified code may not make the distinction between a=b and a:=b.

The rule for definition inference is that you scan through the lines, and a set of already defined symbols is maintained (starting from eqs and defined if provided), and an = op is changed to := if the name on the LHS is not already in the set of symbols already defined.


Gives the C language specifier for numpy data types. For example, numpy.int32 maps to int32_t in C.

Perhaps this method is given somewhere in numpy, but I couldn’t find it.


class brian.experimental.codegen2.CodeGenStateUpdater(group, method, language, clock=None)

State updater using code generation, supports Python, C++, GPU.

Initialised with:

The NeuronGroup that this will be used in.
The integration method, currently one of euler(), rk2() or exp_euler(), but you can define your own too. See make_integration_step() for details.
The Language object.

Creates a Block from the equations and the method, gets a set of Symbol objects from get_neuron_group_symbols(), and defines the symbol _neuron_index as a SliceIndex. Then calls CodeItem.generate() to get the Code object.

Inserts t and dt into the namespace, and _num_neurons and _num_gpu_indices in case they are needed.


class brian.experimental.codegen2.Symbol(name, language)

Base class for all symbols.

Every symbol has attributes name and language which should be a string and Language object respectively. The symbol class should define some or all of the methods below.


Returns the set of dependencies of this symbol, can be overridden.

load(read, write, vectorisable)

Called by resolve(), can be overridden to perform more complicated loading code. By default, returns an empty Block.


Should return True if this symbol is considered to have multiple values, for example if you are iterating over an array like so:

for(int i=0; i<n; i++)
    double &x = arr[i];

Here the symbol x is single-valued and depends on the symbol i which is multi-valued and whose resolution required a loop. By default returns False unless the class has an attribute multiple_values in which case this is returned.


The string that should be used when this symbol is read, by default just the symbol name.


Should return True if the resolution of this symbol will require a loop. The resolve() function uses this to optimise the symbol resolution order.

resolve(read, write, vectorisable, item, namespace)

Creates a modified item in which the symbol has been resolved.

For example, if we started from the expression:

x += 1

and we wanted to produce the following C++ code:

for(int i=0; i<n; i++)
    double &x = __arr_x[i];
    x += 1;

we would need to take the expression x+=1 and embed it inside a loop.

Function arguments:

Whether or not we read the value of the symbol. This is computed by analysing the dependencies by the main resolve() function.
Whether or not we write a value to the symbol.
Whether or not the expression is vectorisable. In Python, we can only vectorise one multi-valued index, so if there are two or more, only the innermost loop will be vectorised.
The code item which needs to be resolved.
The namespace to put data in.

The default implementation first calls update_namespace(), then creates a new Block consisting of the value returned by load(), the item, and the value returned by save(). Finally, this symbol’s name is added to the resolved set for this block.

save(read, write, vectorisable)

Called by resolve(), can be overridden to perform more complicated saving code. By default, returns an empty Block.


Returns True if the language specified at initialisation is supported. By default, checks if the language name is in the class attribute supported_languages (list), however can be overridden.

supported_languages = []
update_namespace(read, write, vectorisable, namespace)

Called by resolve(), can be overridden to modify the namespace, e.g. adding data.


The string that should be used when this symbol is written, by default just the symbol name.

class brian.experimental.codegen2.RuntimeSymbol(name, language)

This Symbol is guaranteed by the context to be inserted into the namespace at runtime and can be used without modification to the name, for example t or dt.


Returns True.

class brian.experimental.codegen2.ArraySymbol(arr, name, language, index=None, array_name=None)

This symbol is used to specify a value taken from an array.

Schematically: name = arr[index].

arr (numpy array)
The numpy array which the values will be taken from.
name, language
The name of the symbol and language.
The index name, by default '_index_'+name.
The name of the array, by default '_arr_'+name.

Introduces a read-dependency on index and array_name.


Read-dependency on index.

load(*args, **kwds)

Method generated by language_invariant_symbol_method().

Languages and methods follow:

load_c(read, write, vectorisable)

Uses CDefineFromArray.

load_python(read, write, vectorisable)

If read is false, does nothing. Otherwise, returns a CodeStatement of the form:

name = array_name[index]
supported_languages = ['python', 'c', 'gpu']
update_namespace(read, write, vectorisable, namespace)

Adds pair (array_name, arr) to namespace.

write(*args, **kwds)

Method generated by language_invariant_symbol_method().

Languages and methods follow:


Returns array_name[index].

class brian.experimental.codegen2.NeuronGroupStateVariableSymbol(group, varname, name, language, index=None)

Symbol for a state variable.

Wraps ArraySymbol.


name, language
Symbol name and language.
The NeuronGroup.
The state variable name in the group.
An index name (or use default of ArraySymbol).
class brian.experimental.codegen2.SliceIndex(name, start, end, language, all=False)

Multi-valued symbol that ranges over a slice.

Schematically: name = slice(start, end)

name, language
Symbol name and language.
The initial value, can be an integer or string.
The final value (not included), can be an integer or string.
Set to True to indicate that the slice covers the whole range possible (small optimisation for Python).
multiple_values = True

Returns True except for Python.

resolve(*args, **kwds)

Method generated by language_invariant_symbol_method().

Languages and methods follow:

resolve_c(read, write, vectorisable, item, namespace)

Returns item embedded in a C for loop.

resolve_gpu(read, write, vectorisable, item, namespace)

If not vectorisable return resolve_c(). If vectorisable we mark it by adding _gpu_vector_index = name and _gpu_vector_slice = (start, end) to the namespace. The GPU code will handle this later on.

resolve_python(read, write, vectorisable, item, namespace)

If vectorisable and all then we simply return item and add name=slice(None) to the namespace.

If vectorisable and not all then we prepend the following statement to item:

name = slice(start, end)

If not vectorisable then we add a for loop over xrange(start, end).

supported_languages = ['python', 'c', 'gpu']
class brian.experimental.codegen2.ArrayIndex(name, array_name, language, array_len=None, index_name=None, array_slice=None)

Multi-valued symbol giving an index that iterates through an array.

Schematically: name = array_name[array_slice]

name, language
Symbol name and language.
The name of the array we iterate through.
The length of the array (int or string), by default has value '_len_'+array_name.
The name of the index into the array, by default has value '_index_'+array_name.
A pair (start, end) giving a slice of the array, if left the whole array will be used.

Dependencies are collected from those arguments that are used (item, array_name, array_len, array_slice).

multiple_values = True

Returns True except for Python.

resolve(*args, **kwds)

Method generated by language_invariant_symbol_method().

Languages and methods follow:

resolve_c(read, write, vectorisable, item, namespace)

Returns a C++ for loop of the form:

for(int index_name=start; index_name<end; index_name++)
    const int name = array_name[index_name];

If defined (start, end)=array_slice otherwise (start, end)=(0, array_len).

resolve_gpu(read, write, vectorisable, item, namespace)

If not vectorisable, use resolve_c(). If vectorisable, we set the following in the namespace:

_gpu_vector_index = index_name
_gpu_vector_slice = (start, end)

Where start and end are as in resolve_c(). This marks that we want to vectorise over this index, and the GPU code will handle this later. Finally, we prepend the item with:

const int name = array_name[index_name];
resolve_python(read, write, vectorisable, item, namespace)

If vectorisable it will prepend one of these two forms to item:

name = array_name
name = array_name[start:end]

(where (start, end) = array_slice if provided).

If not vectorisable, it will return a for loop over either array_name or array_name[start:end]`.

supported_languages = ['python', 'c', 'gpu']
brian.experimental.codegen2.language_invariant_symbol_method(basemethname, langs, fallback=None, doc=None)

Helper function to create methods for Symbol classes.

Sometimes it is clearer to write a separate method for each language the Symbol supports. This function can generate a method that can take any language, and calls the desired method. For example, if you had defined two methods load_python and load_c then you would define the load method as follows:

load = language_invariant_symbol_method('load',
    {'python':load_python, 'c':load_c})

The fallback gives a method to call if no language-specific method was found. A docstring can be provided to doc.

brian.experimental.codegen2.get_neuron_group_symbols(group, language, index='_neuron_index', prefix='')

Returns a dict of NeuronGroupStateVariable from a group.


The group to extract symbols from.
The language to use.
The name of the neuron index, by default _neuron_index.
An optional prefix to add to each symbol.


class brian.experimental.codegen2.CodeGenThreshold(group, inputcode, language, level=0)