Python code printer

[Upabjojr]

The current Python printer prints expressions in a SymPy-style.

It could be useful to have a printer that prints non-SymPy Python code, for example:

• Mod(x, 2) as x % 2
• And(x, y) as x and y instead of SymPy's operator overloaded x & y.

Currently:

• str( ) is supposed to return a valid Python code that is (with some exceptions/caveats) able to reconstruct the SymPy object.
• lambdify( ) returns a lambda objects for optimized operations.

[mohit3011]

@Upabjojr , I would like to work on this issue, could you please refer the starting point

[Upabjojr]

Better discuss this issue a bit before starting, especially to get an idea of what other community members think about it.

This could potentially clash with the lambdify functionality.

[mohit3011]

Yes, you are right.

[asmeurer]

Do you mean the printer in sympy.printing.python? I don't think that is used by anything, except for the python function. It isn't used by lambdify.

I agree it would be better to expand the functionality of this. It would be useful to directly codegen to Python. For instance, one could then subclass this to create a CythonPrinter.

It does tie in pretty closely with the LambdaPrinter. They should definitely share some code. I'm not entirely sure what that should look like. LambdaPrinter is constrained in that it needs to print expressions, whereas the python printer can print entire blocks of code (although most everything in Python can be represented as a single expression).

Maybe @bjodah has some ideas?

[asmeurer]

And I should add that the str printer could be a superclass here, but it shouldn't be considered to be a code generation printer. Aside from what you said that it focuses on SymPy output, it also has a high focus on readability and human consumption, whereas a code generator printer doesn't have that restriction.

[moorepants]

I agree this is needed. I've needed it many times and just hacked around the str printer. I think it should likely be separate than lambdify because lambdify does all these import tricks. It should just be another code printer that ties into the rest of the codegen framework.

[asmeurer]

Also, for what it's worth, the lambdify restriction that it prints an expression is not strictly needed. Right now, lambdify works like

return eval('lambda %s: %s' % (args, lambdarepr(expr)))

(roughly). But it could also work like

ns = {}
exec("""
def f(%s):
    return %s
""" % (args, lambdarepr(expr)), {})
return ns['f']

which would open up the possibility to do things like variable assignment inside of the lambdified expression. And then, in the long run, lambdify would just be a wrapper around the python printer. The LambdaRepr (expression only) printer could be kept for backwards compatibility (and it could be useful for general code printing).

[mohit3011]

@Upabjojr , I read about Lambdify as well as about codegen. I don't know about the difficulty of this issue if it is easy to fix then I would like to work. Could you please refer to the starting point for solving this issue.

[Upabjojr]

@mohit3011 , I would roughly try to see how lambdify works and figure out a way of printing the generated code instead of evaluating it. Not sure about the difficulty.

[mohit3011]

@asmeurer , could you refer to any starting point on this issue and it's difficulty level. Thanks.

[mohit3011]

@Upabjojr , did you look into this issue further ?

[Upabjojr]

did you look into this issue further ?

No.

The way to go is to debug the lambdify function and see if you can reorganize the code to retrieve the expression string. If you feel like doing it, have a try!

[mohit3011]

@Upabjojr , please have a look at #12250

[mohit3011]

@Upabjojr , I looked into lambdify function in lambdify.py and also lambdarepr.py , but I couldn't get anything . Could you please help me

[moorepants]

@mohit3011 I suggest reading the documentation on how printing works: http://docs.sympy.org/latest/modules/printing.html as a start.

[Upabjojr]

@mohit3011 just debug the code, put some breakpoints and explore the local variables.

Once you understand how the code works, it will be easier to purpose an edit.

[bjodah]

Since there is a rudimentary Python code printer now (since gh-12808) should we close this issue? We can open up new issues for the new printer (wishlists and/or bug-reports)