Using (as of now) Undefined Functions in Lambda Functions in Python

Question

I would like to define a lambda function in a different module than it will be executed in. In the module that the lambda will be called, there are methods available that aren't when the lambda is defined. As it is, Python throws an error when the lambda tries to employ those functions.

For example, I have two modules.

lambdaSource.py:

def getLambda():
    return lambda x: squareMe(x)

runMe.py

import lambdaSource

def squareMe(x):
    return x**2

if __name__ == '__main__':
    theLambdaFunc = lambdaSource.getLambda()
    result        = theLambdaFunc(5)

If you run runMe.py, you get a Name Error: NameError: global name 'squareMe' is not defined

The only way I can get around this is to modify the lambda's global variables dictionary at runtime.

theLambdaFunc.func_globals['squareMe'] = squareMe

This example is contrived, but this is the behavior I desire. Can anyone explain why the first example doesn't work? Why 'squareMe' isn't available to the scope of the lambda function? Especially when, if I just defined the lambda below the function squareMe, everything works out okay?

Can't you just make getLambda take an argument and pass in the function, e.g. getLambda(f): return lambda x: f(x)? I can't see why you would need to do it the way you're doing it now. — Marius
– Marius, Commented Jul 21, 2015 at 1:11
To be clear, this example just illustrates the problem. Really, the lambda function comes from a static configuration.ini file, parsed by another python module, and passed to a higher level module for execution. — zachd1_618
– zachd1_618, Commented Jul 21, 2015 at 2:37

Max Noel · Accepted Answer · 2015-07-21 01:05:04Z

3

You're defining getLambda and squareMe in separate modules. The lambdaSource module only sees what's defined in its scope -- that is, everything you define directly in it and everything you import in it.

To use squareMe from getLambda, you need lambdaSource.py to have a from runMe import squareMe statement (and not the other way around as you seem to be doing).

answered Jul 21, 2015 at 1:05

Max Noel

8,9301 gold badge30 silver badges35 bronze badges

Sign up to request clarification or add additional context in comments.

2 Comments

zachd1_618 Over a year ago

That makes sense, but I also would think that with an interpretive language, the lambda would have the scope available to it at the time of execution

Max Noel Over a year ago

That has nothing to do with the way the language is implemented (btw, Python is bytecode-compiled, not interpreted). It's strictly a design decision: the lambda does, in fact, have access to everything in the enclosing scope. It's just that said scope is the module's scope (Python doesn't have a concept of truly global scope, aside from its builtins).

Collectives™ on Stack Overflow

Using (as of now) Undefined Functions in Lambda Functions in Python

1 Answer 1

2 Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

2 Comments

Your Answer

Sign up or log in

Post as a guest

Related