CODEWITHSUNDEEP
pythondictionaryfor-loopkey
idk
idk

Reputation: 63

Difference between iteration over dictionary keys with and without keys() method

What is the difference between for loop in the dictionaries with .keys() method and without .keys() method in Python?

thisdict = {
  "brand": "Ford",
  "model": "Mustang",
  "year": 1964
}
for i in thisdict:
    print(i)
for i in thisdict.keys():
    print(i)

What is the difference?

Upvotes: 6

Views: 1107

Answers (2)

colidyre
colidyre

Reputation: 4676

From the Python documentation:

keys()

    Return a new view of the dictionary’s keys. See the documentation of view objects.

and from the link above:

The objects returned by dict.keys(), dict.values() and dict.items() are view objects. They provide a dynamic view on the dictionary’s entries, which means that when the dictionary changes, the view reflects these changes.

Dictionary views can be iterated over to yield their respective data, and support membership tests [...]

Without keys()-method, you have simply an iterator over the keys available (which is probably preferred), as you can see here (same as second, but without loading and calling keys method):

In [1]: import dis                                                              

In [2]: dis.dis("for i in {None: None}: print(i)")                              
  1           0 LOAD_CONST               0 (None)
              2 LOAD_CONST               0 (None)
              4 BUILD_MAP                1
              6 GET_ITER
        >>    8 FOR_ITER                12 (to 22)
             10 STORE_NAME               0 (i)
             12 LOAD_NAME                1 (print)
             14 LOAD_NAME                0 (i)
             16 CALL_FUNCTION            1
             18 POP_TOP
             20 JUMP_ABSOLUTE            8
        >>   22 LOAD_CONST               0 (None)
             24 RETURN_VALUE

In [3]: dis.dis("for i in {None: None}.keys(): print(i)")                       
  1           0 LOAD_CONST               0 (None)
              2 LOAD_CONST               0 (None)
              4 BUILD_MAP                1
              6 LOAD_METHOD              0 (keys)
              8 CALL_METHOD              0
             10 GET_ITER
        >>   12 FOR_ITER                12 (to 26)
             14 STORE_NAME               1 (i)
             16 LOAD_NAME                2 (print)
             18 LOAD_NAME                1 (i)
             20 CALL_FUNCTION            1
             22 POP_TOP
             24 JUMP_ABSOLUTE           12
        >>   26 LOAD_CONST               0 (None)
             28 RETURN_VALUE

If you only want to iterate over the keys of a dictionary, there is no need to call keys()-method which gives you the dynamic view on the dictionary’s entries.

Upvotes: 5

Leon Matthews
Leon Matthews

Reputation: 100

The keys() method returns a set-like object, which can be useful for calculating intersections with other dictionaries and so on. In your case, as you're just iterating, there is no practical difference at all.

Upvotes: 2

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