Python Forum

I came across the following bug

D = dict()
D = D['spam'] = dict()

I expected that D['spam'] = dict() would create a new dictionary as a value in D, then the D = part would reassign the variable D as this new empty dictionary.

But this is not what happened! Here is a more detailed version

>>> 
>>> D = dict()
>>> X = D
>>> D = D['spam'] = dict()
>>> D
{'spam': {...}}    # Hey! dictionary D is not empty. It has a key 'spam'
>>> D is D['spam']     # What? D['spam'] is D itself! D is a cyclic object that contains itself
True
>>> X is D
False
>>> X
{}     # X is still the empty dictionary that we created at the beginning

Explanation
The dis module explains what's going on

>>> import dis
>>> dis.dis('D = D["spam"] = dict()')
  1           0 LOAD_NAME                0 (dict)
              2 CALL_FUNCTION            0            # The dict() function is called and its result is on top of the stack
              4 DUP_TOP                                       # The top of the stack is duplicated!
              6 STORE_NAME               1 (D)        # the top element is consumed and assigned to D: from left to right
              8 LOAD_NAME                1 (D)
             10 LOAD_CONST               0 ('spam')
             12 STORE_SUBSCR                         # the duplicated top element is consumed and assigned to D['spam']
             14 LOAD_CONST               1 (None)
             16 RETURN_VALUE
>>> 

Conclusion: when we write

a = b = c = d = value

it is equivalent, in this order to

a = value
b = value
c = value
d = value

When b is a['spam'] it is the new a that is used for the second assignment!

What should I have done?

D['spam'] = D = dict()

Proof

>>> D = dict()
>>> X = D
>>> D['spam'] = D = dict()
>>> D
{}
>>> X
{'spam': {}}
>>> X['spam'] is D
True

D['spam'] = D = dict()

The last assignment wins. The last assignment is an empty dict.

EDIT: End it seems that dict() is assigned to D and not to D["spam"]

(Oct-08-2023, 01:03 PM)DeaD_EyE Wrote: [ -> ]EDIT: End it seems that dict() is assigned to D and not to D["spam"]

Yes it is. The assignment does the same as

value = dict()
D['spam']=value
D = value