Quick question about sympify variables

[Andrew Spielberg]

Hi guys,

I have a variable with a period in it (say, 'base.length').  I now wish to sympify an expression, say:

2*base.length

This throws an exception,

AttributeError: 'Symbol' object has no attribute 'length'

due to the period.  Is there a way I can have the sympify read the dot as just part of a variable name here?

-Andy S.

[Jason Moore]

Andy,

The period is an invalid character for Python variables. The period is reserved for accessing attributes and methods of Python objects.

Jason
moorepants.info
+01 530-601-9791

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[Andrew Spielberg]

Hi Jason,

That's true that it's an invalid character for python variables, but not for sympy variables.  For instance, doing:

s = Symbol('base.length')

works just fine.

I was under the impression that sympify's inputs is treated as a string version of a sympy expression, NOT a Python expression.  This seems to be what the documentation implies: http://docs.sympy.org/dev/modules/core.html.

Shouldn't this work?  I feel like either the documentation is misleading/ambiguous or the function is not matching the specification.

Is there a workaround you would recommend that will properly parse periods as Symbol does?

-Andy S.

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[Jason Moore]

Oh I see. I didn't realize you were using sympify. But I don't see anywhere in the docs that indicates that sympify should understand how to deal with variable names that include a period. The function that is failing is:

http://docs.sympy.org/dev/modules/parsing.html#sympy.parsing.sympy_parser.parse_expr

You can customize the parsers if you want to handle different kinds of string input.

Jason
moorepants.info
+01 530-601-9791

To view this discussion on the web visit https://groups.google.com/d/msgid/sympy/CAM6e1JdwJw52%3D%3DqKFWiSVfMstvwAOO-2vWuvfBfgeTjjp2%3DwSA%40mail.gmail.com.

[Aaron Meurer]

You can see if the parser is modifiable. sympify is more or less just a pass through on a Python parser, which maps undefined variable names to Symbols. sympify('2*lambda') doesn't work either, for instance ('lambda' is a reserved Python keyword).

Aaron Meurer

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