"""
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,
DimensionMismatchError,
have_same_dimensions,
get_dimensions,
DIMENSIONLESS,
get_unit_for_display)
__all__ = ['parse_expression_dimensions']
[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__ in [getattr(ast, 'NameConstant', None),
getattr(ast, 'Constant', None)]:
value = expr.value
if value is True or value is False:
return True
elif value is None:
raise ValueError("Do not know how to deal with 'None'")
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(f'Unknown function {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, str):
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(f'Value {name} is not constant')
if not getattr(variables[name], 'scalar', False):
raise SyntaxError(f'Value {name} is not scalar')
return variables[name].get_value()
elif name in ['True', 'False']:
return 1.0 if name == 'True' else 0.0
else:
raise ValueError(f'Unknown identifier {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(f'Do not know how to deal with value {value}')
elif (expr.__class__ is ast.Num or
expr.__class__ is getattr(ast, 'Constant', None)): # Python 3.8
# In Python 3.8, boolean values are represented by Constant, not by
# NameConstant
if expr.n is True or expr.n is False:
return 1.0 if expr.n else 0.0
else:
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 = float(left) / right
elif op == 'FloorDiv':
v = left // right
elif op == 'Pow':
v = left**right
elif op == 'Mod':
v = left % right
else:
raise SyntaxError(f"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(f"Unknown unary operation {op}")
else:
raise SyntaxError(f"Unsupported operation {str(expr.__class__)}")
[docs]def parse_expression_dimensions(expr, variables, orig_expr=None):
"""
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, str):
orig_expr = expr
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 DIMENSIONLESS
else:
raise ValueError(f'Do not know how to handle value {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(f'{name} was used like a variable/constant, but it is a function.',
("<string>",
expr.lineno,
expr.col_offset + 1,
orig_expr)
)
if name in variables:
return get_dimensions(variables[name])
elif name in ['True', 'False']:
return DIMENSIONLESS
else:
raise KeyError(f'Unknown identifier {name}')
elif (expr.__class__ is ast.Num or
expr.__class__ is getattr(ast, 'Constant', None)): # Python 3.8
return DIMENSIONLESS
elif expr.__class__ is ast.BoolOp:
# check that the units are valid in each subexpression
for node in expr.values:
parse_expression_dimensions(node, variables, orig_expr=orig_expr)
# but the result is a bool, so we just return 1 as the unit
return DIMENSIONLESS
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_dimensions(node, variables, orig_expr=orig_expr))
for left_dim, right_dim in zip(subunits[:-1], subunits[1:]):
if not have_same_dimensions(left_dim, right_dim):
left_expr = NodeRenderer().render_node(expr.left)
right_expr = NodeRenderer().render_node(expr.comparators[0])
dim_left = get_dimensions(left_dim)
dim_right = get_dimensions(right_dim)
msg = (f"Comparison of expressions with different units. Expression "
f"'{left_expr}' has unit ({dim_left}), while expression "
f"'{right_expr}' has units ({dim_right}).")
raise DimensionMismatchError(msg)
# but the result is a bool, so we just return 1 as the unit
return DIMENSIONLESS
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(f'Unknown function {expr.func.id}',
("<string>",
expr.lineno,
expr.col_offset + 1,
orig_expr)
)
if not hasattr(func, '_arg_units') or not hasattr(func, '_return_unit'):
raise ValueError(f"Function {expr.func_id} does not specify how it "
f"deals with units.")
if len(func._arg_units) != len(expr.args):
raise SyntaxError(f"Function '{expr.func.id}' was called with "
f"{len(expr.args)} parameters, needs "
f"{len(func._arg_units)}.",
("<string>",
expr.lineno,
expr.col_offset + len(expr.func.id) + 1,
orig_expr))
for idx, (arg, expected_unit) in enumerate(zip(expr.args,
func._arg_units)):
arg_unit = parse_expression_dimensions(arg, variables,
orig_expr=orig_expr)
# A "None" in func._arg_units means: No matter what unit
if expected_unit is None:
continue
# A string means: same unit as other argument
elif isinstance(expected_unit, str):
arg_idx = func._arg_names.index(expected_unit)
expected_unit = parse_expression_dimensions(expr.args[arg_idx],
variables,
orig_expr=orig_expr)
if not have_same_dimensions(arg_unit, expected_unit):
msg = (f'Argument number {idx + 1} for function '
f'{expr.func.id} was supposed to have the '
f'same units as argument number {arg_idx + 1}, but '
f"'{NodeRenderer().render_node(arg)}' has unit "
f'{get_unit_for_display(arg_unit)}, while '
f"'{NodeRenderer().render_node(expr.args[arg_idx])}' "
f'has unit {get_unit_for_display(expected_unit)}')
raise DimensionMismatchError(msg)
elif expected_unit == bool:
if not is_boolean_expression(arg, variables):
rendered_arg = NodeRenderer().render_node(arg)
raise TypeError(f"Argument number {idx + 1} for function "
f"'{expr.func.id}' was expected to be a boolean "
f"value, but is '{rendered_arg}'.")
else:
if not have_same_dimensions(arg_unit, expected_unit):
rendered_arg = NodeRenderer().render_node(arg)
arg_unit_dim = get_dimensions(arg_unit)
expected_unit_dim = get_dimensions(expected_unit)
msg = (f"Argument number {idx+1} for function {expr.func.id} does "
f"not have the correct units. Expression '{rendered_arg}' "
f"has units ({arg_unit_dim}), but " f"should be "
f"({expected_unit_dim}).")
raise DimensionMismatchError(msg)
if func._return_unit == bool:
return DIMENSIONLESS
elif isinstance(func._return_unit, (Unit, int)):
# Function always returns the same unit
return getattr(func._return_unit, 'dim', DIMENSIONLESS)
else:
# Function returns a unit that depends on the arguments
arg_units = [parse_expression_dimensions(arg, variables, orig_expr=orig_expr)
for arg in expr.args]
return func._return_unit(*arg_units).dim
elif expr.__class__ is ast.BinOp:
op = expr.op.__class__.__name__
left_dim = parse_expression_dimensions(expr.left, variables, orig_expr=orig_expr)
right_dim = parse_expression_dimensions(expr.right, variables, orig_expr=orig_expr)
if op in ['Add', 'Sub', 'Mod']:
# dimensions should be the same
if left_dim is not right_dim:
op_symbol = {'Add': '+', 'Sub': '-', 'Mod': '%'}.get(op)
left_str = NodeRenderer().render_node(expr.left)
right_str = NodeRenderer().render_node(expr.right)
left_unit = get_unit_for_display(left_dim)
right_unit = get_unit_for_display(right_dim)
error_msg = (f"Expression '{left_str} {op_symbol} {right_str}' uses "
f"inconsistent units ('{left_str}' has unit "
f"{left_unit}; '{right_str}' "
f"has unit {right_unit}).")
raise DimensionMismatchError(error_msg)
u = left_dim
elif op == 'Mult':
u = left_dim*right_dim
elif op == 'Div':
u = left_dim/right_dim
elif op == 'FloorDiv':
if not (left_dim is DIMENSIONLESS and right_dim is DIMENSIONLESS):
if left_dim is DIMENSIONLESS:
col_offset = expr.right.col_offset + 1
else:
col_offset = expr.left.col_offset + 1
raise SyntaxError("Floor division can only be used on "
"dimensionless values.",
("<string>",
expr.lineno,
col_offset,
orig_expr)
)
u = DIMENSIONLESS
elif op == 'Pow':
if left_dim is DIMENSIONLESS and right_dim is DIMENSIONLESS:
return DIMENSIONLESS
n = _get_value_from_expression(expr.right, variables)
u = left_dim**n
else:
raise SyntaxError(f"Unsupported operation {op}",
("<string>",
expr.lineno,
getattr(expr.left, 'end_col_offset',
len(NodeRenderer().render_node(expr.left))) + 1,
orig_expr)
)
return u
elif expr.__class__ is ast.UnaryOp:
op = expr.op.__class__.__name__
# check validity of operand and get its unit
u = parse_expression_dimensions(expr.operand, variables, orig_expr=orig_expr)
if op == 'Not':
return DIMENSIONLESS
else:
return u
else:
raise SyntaxError(f"Unsupported operation {str(expr.__class__.__name__)}",
("<string>",
expr.lineno,
expr.col_offset + 1,
orig_expr)
)