CODEWITHSUNDEEP
pythonfunctionparametersreferencemutable
Hind Forsum
Hind Forsum

Reputation: 10507

python: how to change the value of function's input parameter?

I tried to modify the value of a string inside a function, like below:

>>> def appendFlag(target, value):
...     target += value
...     target += " "
...
>>> appendFlag
<function appendFlag at 0x102933398>
>>> appendFlag(m,"ok")
>>> m
''

Well, seems the "target" is only changed within the function, but how to make the new value viable outside the function? Thanks.

Upvotes: 8

Views: 42485

Answers (6)

Simon
Simon

Reputation: 1

Make a new class and set the target as the member of class. Then try setattr(class, 'target', value).

Upvotes: 0

negvet
negvet

Reputation: 27

Mutable data types, such as lists or dictionaries, can be changed from within the function without returning:

def duplicate_last(l):
    l.append(l[-1])

l = [1, 2, 3]
duplicate_last(l)

The reason for that is different memory management for mutable and immutable objects in python.

Upvotes: 3

Agus Mathew
Agus Mathew

Reputation: 941

The variables inside the function has got only function scope, if you change the value of the variable from inside the function it wont get reflected outside the function. If you want to change the value of the global variable m, update it from outside the function as follows

 def appendFlag(target, value):
        target += value
        target += " "
        return target
  m=''
  m=appendFlag(m,'ok')

Upvotes: 1

Christian Sloper
Christian Sloper

Reputation: 7510

This is handled in python by returning.

def appendFlag(target, value):
   target += value
   target += " "
   return target

you can use it like this:

m = appendFlag(m,"ok")

you can even return several variables like this:

def f(a,b):
   a += 1
   b += 1
   return a,b

and use it like this:

a,b = f(4,5)

Upvotes: 5

Kylejie
Kylejie

Reputation: 94

You need to use an object that can be modified

>>> m = []
>>> def appendFlag(target, value):
...     target.append(value)
...     target.append(" ")
...
>>> appendFlag(m, "ok")
>>> m
['ok', ' ']

Upvotes: 3

dwagon
dwagon

Reputation: 511

Your function needs to return the new value. The target variable only has scope within the appendFlag function.

def appendFlag(target, value):
   ...
   return target

m = appendFlag(m, "ok")

Upvotes: 0

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