80 lines
2.7 KiB
Python
80 lines
2.7 KiB
Python
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"""
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This module implements the functionality to take any Python expression as a
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string and fix all numbers and other things before evaluating it,
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thus
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1/2
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returns
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Integer(1)/Integer(2)
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We use the ast module for this. It is well documented at docs.python.org.
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Some tips to understand how this works: use dump() to get a nice
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representation of any node. Then write a string of what you want to get,
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e.g. "Integer(1)", parse it, dump it and you'll see that you need to do
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"Call(Name('Integer', Load()), [node], [], None, None)". You do not need
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to bother with lineno and col_offset, just call fix_missing_locations()
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before returning the node.
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"""
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from sympy.core.basic import Basic
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from sympy.core.sympify import SympifyError
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from ast import parse, NodeTransformer, Call, Name, Load, \
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fix_missing_locations, Str, Tuple
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class Transform(NodeTransformer):
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def __init__(self, local_dict, global_dict):
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NodeTransformer.__init__(self)
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self.local_dict = local_dict
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self.global_dict = global_dict
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def visit_Constant(self, node):
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if isinstance(node.value, int):
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return fix_missing_locations(Call(func=Name('Integer', Load()),
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args=[node], keywords=[]))
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elif isinstance(node.value, float):
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return fix_missing_locations(Call(func=Name('Float', Load()),
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args=[node], keywords=[]))
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return node
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def visit_Name(self, node):
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if node.id in self.local_dict:
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return node
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elif node.id in self.global_dict:
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name_obj = self.global_dict[node.id]
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if isinstance(name_obj, (Basic, type)) or callable(name_obj):
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return node
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elif node.id in ['True', 'False']:
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return node
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return fix_missing_locations(Call(func=Name('Symbol', Load()),
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args=[Str(node.id)], keywords=[]))
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def visit_Lambda(self, node):
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args = [self.visit(arg) for arg in node.args.args]
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body = self.visit(node.body)
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n = Call(func=Name('Lambda', Load()),
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args=[Tuple(args, Load()), body], keywords=[])
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return fix_missing_locations(n)
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def parse_expr(s, local_dict):
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"""
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Converts the string "s" to a SymPy expression, in local_dict.
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It converts all numbers to Integers before feeding it to Python and
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automatically creates Symbols.
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"""
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global_dict = {}
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exec('from sympy import *', global_dict)
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try:
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a = parse(s.strip(), mode="eval")
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except SyntaxError:
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raise SympifyError("Cannot parse %s." % repr(s))
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a = Transform(local_dict, global_dict).visit(a)
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e = compile(a, "<string>", "eval")
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return eval(e, global_dict, local_dict)
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