CODEWITHSUNDEEP
python
marlboro
marlboro

Reputation: 264

in python assign a value based on dictionary mapping

I was wondering if someone could explain this to me:

In [400]: poz0=''

In [401]: poz1=''

In [402]: poz={0:poz0, 1:poz1}

In [403]: for i in range(1):
   .....:     poz[i]='some value '+str(i)
   .....:

In [404]:

In [405]: poz[0]
Out[405]: 'some value 0'

In [406]: poz0
Out[406]: ''

I was expecting for poz0 to be == poz[0], same for poz1, but its not. Anyone could explain why?

Thanks

Upvotes: 1

Views: 160

Answers (2)

Diego Navarro
Diego Navarro

Reputation: 9704

The values are different because str instances are built-in immutable objects (numbers, strings, tuples, frozensets). So when you create the dictionary in poz={0:poz0, 1:poz1} you are actually doing the same as poz={0:'', 1:''}.

poz0 is not linked in any way with poz[0], there are different objects.

Update answering the comment:

With a list is not the same behaviour because lists are mutable objects:

In [9]: l = [1,2,3]

In [10]: d = {0: l}
In [12]: d[0]

Out[12]: [1, 2, 3]

In [13]: d[0].append(4)

In [14]: d[0]
Out[14]: [1, 2, 3, 4]

In [15]: l
Out[15]: [1, 2, 3, 4]

Upvotes: 2

Fred Foo
Fred Foo

Reputation: 363767

You reassigned poz[0] to a different object, namely the value of 'some value '+str(0). The assignment operator = does not modify poz[0]'s value but changing the binding in the dict.

Upvotes: 4

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