Hello,
I'm working at fixing issue 888. It is easy to fix but I don't understand why
in sympy/utilities/lambdify.py I found that lambdastr function does use
__str__ for expr.
def lambdastr(args, expr):
"""
Returns a string that can be evaluated to a lambda function.
>>> from sympy import symbols
>>> x,y,z = symbols('xyz')
>>> lambdastr(x, x**2)
'lambda x: (x**2)'
>>> lambdastr((x,y,z), [z,y,x])
'lambda x,y,z: ([z, y, x])'
"""
# Transform everything to strings.
expr = str(expr)
if isinstance(args, str):
pass
elif hasattr(args, "__iter__"):
args = ",".join(str(a) for a in args)
else:
args = str(args)
return "lambda %s: (%s)" % (args, expr)
But in python doc we can read:
(http://docs.python.org/ref/customization.html)
################################
__repr__( self)
Called by the repr() built-in function and by string conversions (reverse
quotes) to compute the ``official'' string representation of an object. If at
all possible, this should look like a valid Python expression that could be
used to recreate an object with the same value (given an appropriate
environment). If this is not possible, a string of the form "<...some useful
description...>" should be returned. The return value must be a string
object. If a class defines __repr__() but not __str__(), then __repr__() is
also used when an ``informal'' string representation of instances of that
class is required.
This is typically used for debugging, so it is important that the
representation is information-rich and unambiguous.
__str__( self)
Called by the str() built-in function and by the print statement to
compute the ``informal'' string representation of an object. This differs
from __repr__() in that it does not have to be a valid Python expression: a
more convenient or concise representation may be used instead. The return
value must be a string object.
################################
As I understand __repr__ should be used to have a pythonic valid expression
and __str__ should be a normal string. But sympy Basic __repr__
implementation returns unicode text with repr_level_2 so we can't use it in
eval().
This is also discussed in issue 697.
I suggest to change this behaviour, so we can make lambdify more robust. Is
someone working at this?
Thank you
Riccardo
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