Is it possible to change a function's default parameters in Python?

Question

In Python, is it possible to redefine the default parameters of a function at runtime?

I defined a function with 3 parameters here:

def multiplyNumbers(x,y,z):
    return x*y*z

print(multiplyNumbers(x=2,y=3,z=3))

Next, I tried (unsuccessfully) to set the default parameter value for y, and then I tried calling the function without the parameter y:

multiplyNumbers.y = 2;
print(multiplyNumbers(x=3, z=3))

But the following error was produced, since the default value of y was not set correctly:

TypeError: multiplyNumbers() missing 1 required positional argument: 'y'

Is it possible to redefine the default parameters of a function at runtime, as I'm attempting to do here?

Setting the attribute y on the function also doesn't change the default parameter for y. — abarnert
– abarnert, Commented Jul 13, 2013 at 0:13

RiaD · Accepted Answer · 2020-05-24 16:49:46Z

45

Just use functools.partial

 multiplyNumbers = functools.partial(multiplyNumbers, y = 42)

One problem here: you will not be able to call it as multiplyNumbers(5, 7, 9); you should manually say y=7

If you need to remove default arguments I see two ways:

Store original function somewhere

oldF = f
f = functools.partial(f, y = 42)
//work with changed f
f = oldF //restore

use partial.func
```
f = f.func //go to previous version.
```

edited May 24, 2020 at 16:49

answered Jul 13, 2013 at 0:11

RiaD

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13 Comments

RiaD Over a year ago

@joshlf13, you may be in static languages too.

joshlf Over a year ago

You mean may be able to do something like that code snippet? I suppose it'd be possible with a powerful enough type system, but I don't know many languages like that. Many functional languages, I suppose, but I meant imperative (though I should've said that explicitly).

joshlf Over a year ago

Actually, come to think of it, that's basically like currying, except without needing to have your variables ordered.

RiaD Over a year ago

@AndersonGreen, I did understand. I mean, I don't know how to do it directly but you may do it as following oldF = f; f = partials(...); ... ; f = oldF

RiaD Over a year ago

@AndersonGreen, added some info

|

abarnert · Accepted Answer · 2013-07-13 00:25:17Z

21

Technically, it is possible to do what you ask… but it's not a good idea. RiaD's answer is the Pythonic way to do this.

In Python 3:

>>> def f(x=1, y=2, z=3):
...     print(x, y, z)
>>> f()
1 2 3
>>> f.__defaults__ = (4, 5, 6)
4 5 6

As with everything else that's under the covers and hard to find in the docs, the inspect module chart is the best place to look for function attributes.

The details are slightly different in Python 2, but the idea is the same. (Just change the pulldown at the top left of the docs page from 3.3 to 2.7.)

If you're wondering how Python knows which defaults go with which arguments when it's just got a tuple… it just counts backward from the end (or the first of *, *args, **kwargs—anything after that goes into the __kwdefaults__ dict instead). f.__defaults = (4, 5) will set the defaults to y and z to 4 and 5, and with default for x. That works because you can't have non-defaulted parameters after defaulted parameters.

There are some cases where this won't work, but even then, you can immutably copy it to a new function with different defaults:

>>> f2 = types.FunctionType(f.__code__, f.__globals__, f.__name__,
...                         (4, 5, 6), f.__closure__)

Here, the types module documentation doesn't really explain anything, but help(types.FunctionType) in the interactive interpreter shows the params you need.

The only case you can't handle is a builtin function. But they generally don't have actual defaults anyway; instead, they fake something similar in the C API.

edited Jul 13, 2013 at 0:25

answered Jul 13, 2013 at 0:15

abarnert

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5 Comments

Anderson Green Over a year ago

You mentioned that it's "not a good idea" to change the default parameters of a function at runtime. Have you noticed any disadvantages of this approach? (I'm not yet aware of any).

abarnert Over a year ago

@AndersonGreen: Well, there's the obvious disadvantage that it makes it harder for you, other humans, linters, IDEs, reflection-based tools like pickle, etc. to understand your code. And you have to fully understand the rules about args/params to be sure you're doing it right. Then there's the fact that you can't make it work generally without writing code that can deal with regular functions, weird immutable functions (which need to construct a new function out of the code), and builtin functions (which you need to wrap, with a partial or a new function that calls it).

Diptangsu Goswami Over a year ago

Can we do something like f.__defaults__ = (4, 5, 6) from inside a lambda function? @abarnert

Miriam Over a year ago

@DiptangsuGoswami lambda doesn't allow assignment, although assignment expressions in 3.8 will make that technically possible.

StayFoolish Over a year ago

Interesting, but can you modify default values inside of the function definition? The reason why I ask this is because I have two versions of a library, one is build with foo = true, the other is with foo = false. So, for my function f(a, b=1) return a+b; I want the default value of b =2 if foo = true; if foo = false, it should default to 1.

william_grisaitis · Accepted Answer · 2021-07-02 22:16:31Z

5

yes, you can accomplish this by modifying the function's func.__defaults__ tuple

that attribute is a tuple of the default values for each argument of the function.

for example, to make pandas.read_csv always use sep='\t', you could do:

import inspect

import pandas as pd

default_args = inspect.getfullargspec(pd.read_csv).args
default_arg_values = list(pd.read_csv.__defaults__)
default_arg_values[default_args.index("sep")] = '\t'
pd.read_csv.__defaults__ = tuple(default_arg_values)

answered Jul 2, 2021 at 22:16

william_grisaitis

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Comments

Francisco Meza · Accepted Answer · 2013-07-13 00:20:21Z

1

use func_defaults as in

def myfun(a=3):
    return a

myfun.func_defaults = (4,)
b = myfun()
assert b == 4

check the docs for func_defaults here

UPDATE: looking at RiaD's response I think I was too literal with mine. I don't know the context from where you're asking this question but in general (and following the Zen of Python) I believe working with partial applications is a better option than redefining a function's defaults arguments

edited Jul 13, 2013 at 0:20

answered Jul 13, 2013 at 0:14

Francisco Meza

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2 Comments

RiaD Over a year ago

I didn't manage to set it to second of 3 variables. Only works for last for me.

abarnert Over a year ago

@RiaD: Function defaults have to be a final sequence of parameters; just as you can't write def foo(x, y=2, z):, you can't change the func_defaults/__defaults__ to just cover y.

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