Passing function parameter name to another function in Python

Question

Given a simple function:

def A(a = 1, b = 2):
    return a+b

I want to write another function to change the default parameter value, for either a or b. And user can specify the parameter to change by setting var = a or var = b. For example:

def B(var = 'a', new_value = 10):
    temp_dict = {var:new_value}
    ans = A(var)
    return ans

or

def B(var = 'a', new_value = 10):
    ans = A(var = new_value)
    return ans

In function def B(), after setting var = a and var = new_value = 10, I expect A(var = new_value) to achieve the same effect as A(a = 10). Do you know the correct way to write function def B()? Thanks.

Moinuddin Quadri · Accepted Answer · 2017-09-27 18:23:13Z

5

You are almost there. From your B() function, while making the call to A(), you need to unpack the temp_dict and pass it as an argument to A(). See below:

>>> def A(a = 1, b = 2):
...     return a+b
...

>>> def B(var = 'a', new_value = 10):
...     temp_dict = {var:new_value}
...     ans = A(**temp_dict)
        #        ^ unpack the dict, and pass it as an argument
...     return ans
...

>>> B()
12

For more details on how this ** works with a dict, please take a look at:

Python: *args and **kwargs?

What does ** (double star/asterisk) and * (star/asterisk) do for parameters?

edited Sep 27, 2017 at 18:23

answered Sep 27, 2017 at 18:17

Moinuddin Quadri

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Comments

JL Peyret · Accepted Answer · 2017-09-28 02:27:36Z

I took the liberty of interpreting a bit what the OP said he wanted, i.e. change the default parameter value, for either a or b. So what I did was to return a transformed function A with either the a or b defaults changed via a partial:

from functools import partial

def B3(var ="a", new_value=10):
    return partial(A, **{var:new_value})

sample outputs:

(Pdb) f = B3("a",10)
(Pdb) f()
12
(Pdb) f = B3("b",10)
(Pdb) f()
11
(Pdb) f(a=10)
20
(Pdb) f(b=13)
14
(Pdb) f(a=5,b=5)
10

That is different from the 2nd half of the request however, that of having something based on B(var="a",new_value=10) as function signature.

The only thing is, it chokes happily if you don't use keyword parameters:

(Pdb) f(7,7)
*** TypeError: A() got multiple values for keyword argument 'b'

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