bug in lambdify functionality with trigonometric functions?

[Quirine]

Dear Sympy community,

 

The following line of code gives:

>>> from sympy import *

>>> x,y=symbols('x y')

>>> f=lambdify(x,y*cos(x))

>>> f(1)

0.54030230586814*y

works fine, but interchanging x and y:

>>> f=lambdify(x,x*cos(y))

>>> f(1)

Traceback (most recent call last):

  File "<stdin>", line 1, in <module>

  File "<string>", line 1, in <lambda>

  File "/Users/krol/Library/Enthought/Canopy_64bit/User/lib/python2.7/site-packages/sympy/core/expr.py", line 225, in __float__

    raise TypeError("can't convert expression to float")

TypeError: can't convert expression to float

gives me a headache, 

Does someone know a solution to this?

kind Regards,

Quirine

[James Crist]

`lambdify` is intended for numeric evaluation (but can be made to evaluate symbolically). By default, the functions `sin`, `cos`, etc... are pulled from `math` or `numpy`, which expect floats (and if not given a float, they attempt to convert to float).

What's happening here, is you're passing in a number for `x`, but `y` is still a symbol, and is enclosed in the function as a local. `lambdify` doesn't know this, and uses `math.cos` by default, which causes the error. Using the `modules` kwarg, you can tell lambdify to use `sympy.cos` instead.

>>> f = lambdify(x, x*cos(y), modules='sympy')
>>> f(1)
cos(y)

You can also pass a dictionary of {function_name_as_string: function} to modules, if you want finer control:

>>> f = lambdify(x, x*cos(y), modules={'cos': sympy.cos})
>>> f(1)
cos(y)

[Aaron Meurer]

Or it's possible that you meant to do lambdify((x, y), x*cos(y)) to create a two-argument function.

Aaron Meurer

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