How to name variables with commas?

[Kevin Hunter]

Hullo Sympy Group,

I'm wrapping some Sympy variables into a group via a class object.  The object is attached to another object, and from here, I'm able to collect the name of the group (via __setattr__).  Thus, I can have constructs like this:

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M.X = Var()        # Var is the wrapper class that holds a group variables.

M.X[1,2,'a'] = 1   # automatically create variable

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That automatically creates the variable 'X[1,2,a]' via the Var.__getitem__.  To get the name with commas, I have to do this within the __getitem__ function:

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1. self.m_varsInUse += 1

2. vname = '%s[%d]' % ( self.m_name, self.m_varsInUse )

3. var = symbols( vname, **assumptions )

4. var.name = '%s(%s)' % (self.m_name, str(index).replace(' ', ''))

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Lines 1 and 2 guarantee a new variable, so that Sympy doesn't returned a cached copy of an already created variable, and line 4 puts in a name that potentially has commas.  What I'd like to be able to do is forgo lines one and two, like something akin to this:

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vname = '%s(%s)' % (self.m_name, str(index).replace(' ', ''))

var = symbols( vname, **assumptions )

-----

This seems to return names like 'X[1', '2', and 'a]'.  I'd hazard splitting based on commas is correct behavior, but I'm wondering if there's something I can pass to the constructor that says "No, the commas are part of the name."

Thanks,

Kevin

[Aaron Meurer]

Maybe I'm misunderstanding what you're doing, but if you just want
exactly one Symbol, why don't you just do var = Symbol(vname,
**assumptions)?

Aaron Meurer

[Kevin Hunter]

Err, is this perhaps a bug that has been fixed since the 0.7.1 release?  When I do that, vname can't have commas, else it creates multiple variables:

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# For a mnemonic, here is an example variable from a model with which I'm

# working.  The variable format is <VarName>[year, tech, vintage].

>>> from sympy import *

>>> assumptions = dict()

>>> index = (2010, 'Prius', 2008)

>>> vname = 'Car[%s]' % ','.join(str(i) for i in index)

     # I'm looking to create _single_ variable

>>> v = symbols( vname, **assumptions )

     # but instead I get three

>>> type( v )

<type 'tuple'>

>>> len( v )

3

>>> print "Variable 1: '%s'" % v[0]

Variable 1: 'Car[2010'

>>> print "Variable 2: '%s'" % v[1]

Variable 2: 'Prius'

>>> print "Variable 3: '%s'" % v[2]

Variable 3: '2008]'

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The only way that I currently know how to get a single variable with commas in the name is by updating it's name after it's been created:

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>>> v = symbols( 'some_unique_string', **assumptions )

>>> v.name = vname

>>> type( v )

<class 'sympy.core.symbol.Symbol'>

>>> len( v )

Traceback (most recent call last):

  File "<stdin>", line 1, in <module>

TypeError: object of type 'Symbol' has no len()

>>> print "Variable 4: '%s'" % v

Variable 4: 'X[2010,Prius,2008]'

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Does that help elucidate what I'm asking?  What I'm looking for is more direct method to create variables that have names with commas.

[Aaron Meurer]

It works in 0.7.1:

In [1]: Symbol('x, y')
Out[1]: x, y

In [2]: type(Symbol('x, y'))
Out[2]: sympy.core.symbol.Symbol

In [10]: Symbol("Car[2010, 'prius', 2008]")
Out[10]: Car[2010, 'prius', 2008]

In [11]: type(Symbol("Car[2010, 'prius', 2008]"))
Out[11]: sympy.core.symbol.Symbol

By the way, changing the name of a Symbol after it is created is a bad
idea. It will cause it to compare equal to other symbols of the same
name, but the Symbol's hash will not be updated.

Aaron Meurer

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[Kevin Hunter]

Mea culpa.  I see now the difference you were trying to impart: Symbol (the class) vs symbols (the function).  Pardon me as I misstep more than a few times while learning Sympy.

[Aaron Meurer]

No worries. Let us know if you have any other questions.

Aaron Meurer

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