Python: Substitute variable values before passing to another function

Question

I want to determine whether the input string is a valid function name or not. Is there any way to substitute the value of the variable before being passed to isfunction call ?

#!/usr/bin/python
def testFunc():
    print "Hello world!";
    return;

myString = "testFunc";
isfunction(testFunc); // This returns **True**
isfunction(myString); // This returns **False**

Do you mean that the string is a valid function name, or the variable is a function? — khelwood
– khelwood, Commented Sep 3, 2015 at 13:06
@khelwood: I want to know the value present inside myString is valid or not — Ashwin
– Ashwin, Commented Sep 3, 2015 at 13:09
@Ashwin Then why should isfunction(myString) return false when myString is "testFunc"? — khelwood
– khelwood, Commented Sep 3, 2015 at 14:08

Maroun · Accepted Answer · 2015-09-03 13:09:04Z

1

One way of doing that is using eval, which interprets string as code:

try:
    eval(myString)
except NameError:
    # not a function

answered Sep 3, 2015 at 13:09

Maroun

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1 Comment

Rob Foley Over a year ago

This would execute any code that the string contains, and could cause far more errors than NameError can capture.

Rob Foley · Accepted Answer · 2015-09-03 13:28:58Z

1

Assuming you want to check to see if there exists a loaded function with You could try this:

try:
    if hasattr(myString, '__call__'):
        func = myString
    elif myString in dir(__builtins__):
        func = eval(myString)
    else:
        func = globals()[myString]

except KeyError:
    #this is the fail condition

# you can use func()

The first if is actually unnecessary if you will always guarantee that myString is actually a string and not a function object, I just added it to be safe.

In any case, if you actually plan on executing these functions, I'd tread carefully. Executing arbitrary functions can be risky business.

EDIT:

I added another line to be a bit more sure we don't actually execute code unless we want to. Also changed it so that it is a bit neater

edited Sep 3, 2015 at 13:28

answered Sep 3, 2015 at 13:15

Rob Foley

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2 Comments

Ashwin Over a year ago

hasattr returns False because it considers myString as String variable

Rob Foley Over a year ago

You are correct. Which is why the other conditions exist.

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