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local/global variables in functions
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local/global variables in functions
#1
x = 1


def f1(y):
    y += 1
    return y


x1 = f1(x)
print('x = ', x, '  x1 = ', x1)
answer:
x = 1 x1 = 2

x_list = ['1','2']


def f2(y_list):
    y_list[0] = '0'
    return y_list


x1_list = f2(x_list)
print('x_list = ', x_list, '  x1_list = ', x1_list)
answer:
x_list = ['0', '2'] x1_list = ['0', '2']

why in the first case the global variable is preserved and not in the second case?
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#2
This isn't a matter of global versus local. Rather, it's a matter of Python's sequence implementation. Sequences, such as a list, store values by reference to a memory location. As a result, a list is basically a pointer used in other languages. This enables changes to a list made within a function to change the list outside the function.
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#3
(Apr-08-2020, 01:59 PM)stullis Wrote: This isn't a matter of global versus local. Rather, it's a matter of Python's sequence implementation. Sequences, such as a list, store values by reference to a memory location. As a result, a list is basically a pointer used in other languages. This enables changes to a list made within a function to change the list outside the function.

variables also use a pointer to a certian memory location. why should there be a different approach in both cases?
though more than discussing the philosofy around it, I would prefer to know the way around and understand if there is a 'clean' way to preserve the content of lists, when passing them to a function.
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#4
Lists are mutable, integers are not. If you want an immutable sequence, use a tuple.
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#5
it is a nice suggestion but does not solve the problem I was addressing.
The list is created while the program is running.
Is there away to creat a new list that uses another position in memory?

ok, it can be converted. Fine!
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#6
You can make a copy of the list in your function
def f2(y_list):
    new_list = list(y_list)
    new_list[0] = '0'
    return new_list
Reply
#7
A nice video about this from PyCon: https://www.youtube.com/watch?v=_AEJHKGk9ns
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