'''
AST parsing based analysis of expressions
'''
import ast
from brian2.core.functions import Function
from brian2.parsing.rendering import NodeRenderer
from brian2.units.fundamentalunits import (Unit, get_unit_fast,
DimensionMismatchError,
have_same_dimensions,
get_dimensions
)
__all__ = ['is_boolean_expression',
'parse_expression_unit',]
[docs]def is_boolean_expression(expr, variables):
'''
Determines if an expression is of boolean type or not
Parameters
----------
expr : str
The expression to test
variables : dict-like of `Variable`
The variables used in the expression.
Returns
-------
isbool : bool
Whether or not the expression is boolean.
Raises
------
SyntaxError
If the expression ought to be boolean but is not,
for example ``x<y and z`` where ``z`` is not a boolean variable.
Notes
-----
We test the following cases recursively on the abstract syntax tree:
* The node is a boolean operation. If all the subnodes are boolean
expressions we return ``True``, otherwise we raise the ``SyntaxError``.
* The node is a function call, we return ``True`` or ``False`` depending
on whether the function description has the ``_returns_bool`` attribute
set.
* The node is a variable name, we return ``True`` or ``False`` depending
on whether ``is_boolean`` attribute is set or if the name is ``True`` or
``False``.
* The node is a comparison, we return ``True``.
* The node is a unary operation, we return ``True`` if the operation is
``not``, otherwise ``False``.
* Otherwise we return ``False``.
'''
# If we are working on a string, convert to the top level node
if isinstance(expr, str):
mod = ast.parse(expr, mode='eval')
expr = mod.body
if expr.__class__ is ast.BoolOp:
if all(is_boolean_expression(node, variables)
for node in expr.values):
return True
else:
raise SyntaxError("Expression ought to be boolean but is not (e.g. 'x<y and 3')")
elif expr.__class__ is getattr(ast, 'NameConstant', None):
value = expr.value
if value is True or value is False:
return True
else:
raise ValueError('Do not know how to deal with value %s' % value)
elif expr.__class__ is ast.Name:
name = expr.id
if name in variables:
return variables[name].is_boolean
else:
return name == 'True' or name == 'False'
elif expr.__class__ is ast.Call:
name = expr.func.id
if name in variables and hasattr(variables[name], '_returns_bool'):
return variables[name]._returns_bool
else:
raise SyntaxError('Unknown function %s' % name)
elif expr.__class__ is ast.Compare:
return True
elif expr.__class__ is ast.UnaryOp:
return expr.op.__class__.__name__ == 'Not'
else:
return False
def _get_value_from_expression(expr, variables):
'''
Returns the scalar value of an expression, and checks its validity.
Parameters
----------
expr : str or `ast.Expression`
The expression to check.
variables : dict of `Variable` objects
The information about all variables used in `expr` (including `Constant`
objects for external variables)
Returns
-------
value : float
The value of the expression
Raises
------
SyntaxError
If the expression cannot be evaluated to a scalar value
DimensionMismatchError
If any part of the expression is dimensionally inconsistent.
'''
# If we are working on a string, convert to the top level node
if isinstance(expr, basestring):
mod = ast.parse(expr, mode='eval')
expr = mod.body
if expr.__class__ is ast.Name:
name = expr.id
if name in variables:
if not getattr(variables[name], 'constant', False):
raise SyntaxError('Value %s is not constant' % name)
if not getattr(variables[name], 'scalar', False):
raise SyntaxError('Value %s is not scalar' % name)
return variables[name].get_value()
elif name in ['True', 'False']:
return 1.0 if name == 'True' else 0.0
else:
raise ValueError('Unknown identifier %s' % name)
elif expr.__class__ is getattr(ast, 'NameConstant', None):
value = expr.value
if value is True or value is False:
return 1.0 if value else 0.0
else:
raise ValueError('Do not know how to deal with value %s' % value)
elif expr.__class__ is ast.Num:
return expr.n
elif expr.__class__ is ast.BoolOp:
raise SyntaxError('Cannot determine the numerical value for a boolean operation.')
elif expr.__class__ is ast.Compare:
raise SyntaxError('Cannot determine the numerical value for a boolean operation.')
elif expr.__class__ is ast.Call:
raise SyntaxError('Cannot determine the numerical value for a function call.')
elif expr.__class__ is ast.BinOp:
op = expr.op.__class__.__name__
left = _get_value_from_expression(expr.left, variables)
right = _get_value_from_expression(expr.right, variables)
if op=='Add' or op=='Sub':
v = left + right
elif op=='Mult':
v = left * right
elif op=='Div':
v = left / right
elif op=='Pow':
v = left**right
elif op=='Mod':
v = left % right
else:
raise SyntaxError("Unsupported operation "+op)
return v
elif expr.__class__ is ast.UnaryOp:
op = expr.op.__class__.__name__
# check validity of operand and get its unit
v = _get_value_from_expression(expr.operand, variables)
if op=='Not':
raise SyntaxError(('Cannot determine the numerical value '
'for a boolean operation.'))
if op=='USub':
return -v
else:
raise SyntaxError('Unknown unary operation ' + op)
else:
raise SyntaxError('Unsupported operation ' + str(expr.__class__))
[docs]def parse_expression_unit(expr, variables):
'''
Returns the unit value of an expression, and checks its validity
Parameters
----------
expr : str
The expression to check.
variables : dict
Dictionary of all variables used in the `expr` (including `Constant`
objects for external variables)
Returns
-------
unit : Quantity
The output unit of the expression
Raises
------
SyntaxError
If the expression cannot be parsed, or if it uses ``a**b`` for ``b``
anything other than a constant number.
DimensionMismatchError
If any part of the expression is dimensionally inconsistent.
'''
# If we are working on a string, convert to the top level node
if isinstance(expr, basestring):
mod = ast.parse(expr, mode='eval')
expr = mod.body
if expr.__class__ is getattr(ast, 'NameConstant', None):
# new class for True, False, None in Python 3.4
value = expr.value
if value is True or value is False:
return Unit(1)
else:
raise ValueError('Do not know how to handle value %s' % value)
if expr.__class__ is ast.Name:
name = expr.id
# Raise an error if a function is called as if it were a variable
# (most of the time this happens for a TimedArray)
if name in variables and isinstance(variables[name], Function):
raise SyntaxError('%s was used like a variable/constant, but it is '
'a function.' % name)
if name in variables:
return variables[name].unit
elif name in ['True', 'False']:
return Unit(1)
else:
raise KeyError('Unknown identifier %s' % name)
elif expr.__class__ is ast.Num:
return get_unit_fast(1)
elif expr.__class__ is ast.BoolOp:
# check that the units are valid in each subexpression
for node in expr.values:
parse_expression_unit(node, variables)
# but the result is a bool, so we just return 1 as the unit
return get_unit_fast(1)
elif expr.__class__ is ast.Compare:
# check that the units are consistent in each subexpression
subexprs = [expr.left]+expr.comparators
subunits = []
for node in subexprs:
subunits.append(parse_expression_unit(node, variables))
for left, right in zip(subunits[:-1], subunits[1:]):
if not have_same_dimensions(left, right):
msg = ('Comparison of expressions with different units. Expression '
'"{}" has unit ({}), while expression "{}" has units ({})').format(
NodeRenderer().render_node(expr.left), get_dimensions(left),
NodeRenderer().render_node(expr.comparators[0]), get_dimensions(right))
raise DimensionMismatchError(msg)
# but the result is a bool, so we just return 1 as the unit
return get_unit_fast(1)
elif expr.__class__ is ast.Call:
if len(expr.keywords):
raise ValueError("Keyword arguments not supported.")
elif getattr(expr, 'starargs', None) is not None:
raise ValueError("Variable number of arguments not supported")
elif getattr(expr, 'kwargs', None) is not None:
raise ValueError("Keyword arguments not supported")
func = variables.get(expr.func.id, None)
if func is None:
raise SyntaxError('Unknown function %s' % expr.func.id)
if not hasattr(func, '_arg_units') or not hasattr(func, '_return_unit'):
raise ValueError(('Function %s does not specify how it '
'deals with units.') % expr.func.id)
if len(func._arg_units) != len(expr.args):
raise SyntaxError('Function %s was called with %d parameters, '
'needs %d.' % (expr.func.id,
len(expr.args),
len(func._arg_units)))
for idx, (arg, expected_unit) in enumerate(zip(expr.args,
func._arg_units)):
# A "None" in func._arg_units means: No matter what unit
if expected_unit is None:
continue
elif expected_unit == bool:
if not is_boolean_expression(arg, variables):
raise TypeError(('Argument number %d for function %s was '
'expected to be a boolean value, but is '
'"%s".') % (idx + 1, expr.func.id,
NodeRenderer().render_node(arg)))
else:
arg_unit = parse_expression_unit(arg, variables)
if not have_same_dimensions(arg_unit, expected_unit):
msg = ('Argument number {} for function {} does not have the '
'correct units. Expression "{}" has units ({}), but '
'should be ({}).').format(
idx+1, expr.func.id,
NodeRenderer().render_node(arg),
get_dimensions(arg_unit), get_dimensions(expected_unit))
raise DimensionMismatchError(msg)
if func._return_unit == bool:
return Unit(1)
elif isinstance(func._return_unit, (Unit, int)):
# Function always returns the same unit
return get_unit_fast(func._return_unit)
else:
# Function returns a unit that depends on the arguments
arg_units = [parse_expression_unit(arg, variables)
for arg in expr.args]
return func._return_unit(*arg_units)
elif expr.__class__ is ast.BinOp:
op = expr.op.__class__.__name__
left = parse_expression_unit(expr.left, variables)
right = parse_expression_unit(expr.right, variables)
if op=='Add' or op=='Sub':
u = left+right
elif op=='Mult':
u = left*right
elif op=='Div':
u = left/right
elif op=='Pow':
if have_same_dimensions(left, 1) and have_same_dimensions(right, 1):
return get_unit_fast(1)
n = _get_value_from_expression(expr.right, variables)
u = left**n
elif op=='Mod':
u = left % right
else:
raise SyntaxError("Unsupported operation "+op)
return u
elif expr.__class__ is ast.UnaryOp:
op = expr.op.__class__.__name__
# check validity of operand and get its unit
u = parse_expression_unit(expr.operand, variables)
if op=='Not':
return get_unit_fast(1)
else:
return u
else:
raise SyntaxError('Unsupported operation ' + str(expr.__class__))