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How to pass variable by reference in Python?

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How to pass variable by reference in Python?
posted May 6, 2014 by Satish Mishra

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2 Answers

+1 vote
Best answer

Python provides neither pass-by-reference nor pass-by-value argument passing. Please read this for an explanation of why people sometimes think that it does, and what Python actually does instead:

To get an effect *similar* to pass-by-reference, you can wrap your variable in a list, and then only operate on the list item. For example:

one = [1]
two = [2]

def swap(a, b):
 a[0], b[0] = b[0], a[0]

swap(one, two)
print one[0], two[0]
=> will print "2 1"

But note carefully that in the swap function I do not assign directly to the arguments a and b, only to their items a[0] and b[0]. If you assign directly to a and b, you change the local variables.

# This does not work.
def swap(a, b):
 a, b = b, a

Rather than trying to fake pass-by-reference semantics, it is much better to understand Python's capabilities and learn how to use it to get the same effect. For example, instead of writing a swap procedure, it is much simpler to just do this:

one = 1
two = 2
one, two = two, one
print one, two
=> will print "2 1"
answer May 6, 2014 by Amit Parthsarthi
+1 vote
answer May 6, 2014 by Sheetal Chauhan
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