How to bring variables into functions - Python

Question

I am having an issue where I want to bring a variable into a function by name.

I have tried this so far:

x = 5
def func(y = x):
    return y ** 2

>>>print func()
25
>>>x = 4
>>>print func()
25

However if I try to edit that input variable after I input the code the number the function detects the old input so it would still print 25 as oppose to the new input squared.

Is there a cleaner way to pull a variable into a function that actually works?

score 0 · Accepted Answer · edited May 23 '17 at 12:29

Default functions values are calculated during function creation time, so even if you change the value of x it is not going to be changed in the function.

From docs:

Default parameter values are evaluated when the function definition is executed. This means that the expression is evaluated once, when the function is defined, and that the same “pre-computed” value is used for each call.

Read: Using mutable objects as default value can lead to unexpected results.

If you want to access a global value then simply do:

def func():
    return x**2

Or better pass it explicitly:

def func(y):
   return y**2

Another reason why not to use global variables inside function:

>>> x = 10
>>> def func():
...     print x
...     x = 5
...     
>>> func()
Traceback (most recent call last):
    func()
  File "<ipython-input-5-ab38e6cadf6b>", line 2, in func
    print x
UnboundLocalError: local variable 'x' referenced before assignment

Using classes:

class A:
    x = 5
    @staticmethod
    def func():
        return A.x**2
...     
>>> A.func()
25
>>> A.x = 10
>>> A.func()
100

Thanks, I didn't know you could reference global variables in functions. Would +rep but I can't. — deadfire19, Nov 02 '13 at 15:03
@deadfire19 Yes you can access global variables inside a function, but usually its not recommended. It's better to pass it explicitly. — Ashwini Chaudhary, Nov 02 '13 at 15:05

score 0 · Answer 2 · answered Nov 02 '13 at 14:56

0

Just reference the global variable directly, instead of using it as the default value of an argument.

def func():
    return x ** 2

answered Nov 02 '13 at 14:56

Barmar

741,623
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Ah, sorry, first time using this site. – deadfire19 Nov 02 '13 at 15:05

score 0 · Answer 3 · answered Nov 02 '13 at 14:57

0

The default value of y is set during function definition, not when running it. So default value for y is set to value of x on def line.

This should work:

def func(y=None):
    if y is None:
        y = x
    return y ** 2

answered Nov 02 '13 at 14:57

warvariuc

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score 0 · Answer 4 · answered Nov 02 '13 at 15:07

Generally in python Keyword Arguments are always assigned a value and you cant change that assigned value . So in your code

x=5
def func(y = x):
    return y ** 2
print func()
x=4
print func()

For the keyword argument y the value of x i.e 5 is assigned and we cant change that value. So even if you change the value of x you will get the same output as 25

To solve this the function might directly access the global variable and return the value like the below code

x=5
def func():
   return x ** 2
print func()
x=4
print func()

score 0 · Answer 5 · answered Nov 02 '13 at 15:15

0

This is the correct way to do this

x=5
def foo():
    global x
    return x**2

Or you can simply do it like this

x=5
def foo():
    return x**2

In this case the same global variable is referenced if it exists.

answered Nov 02 '13 at 15:15

Sudhanshu Mishra

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How to bring variables into functions - Python

5 Answers5