What is better for dict value assign a nonexistent key?-CodePudding

Suppose that I want to add a value to a dict, but I'm not sure if the key exists. What would be the better way (performance matter)? If there is some better way...

my_dict['key'] = my_dict.get('key', new_value) # I like this!

if 'key' not in my_dict:
    my_dict['key'] = new_value

CodePudding user response：

There's a built-in dict method for this:

my_dict.setdefault('key', new_value)

It returns the resulting value (i.e. the existing value if there was one, or the newly set one if there wasn't), which is useful in expressions like:

my_dict.setdefault('key', []).append('foo')

CodePudding user response：

Simply assign the new value like so:

my_dict['key'] = new_value

The old value, if exists, will be overwritten. If it does not exist, the key will be created and the new value will be assigned.

CodePudding user response：

you can use dis(Disassembler for Python bytecode) and check run-time. best approach base run-time and dis is if 'key' not in my_dict: my_dict['key'] = 'new_value', because first and second approaches have CALL_METHOD but third approach doesn't have CALL_METHOD. (Benchmark on colab)

my_dict = {i:i for i in range(10_000_000)}
%timeit my_dict['key'] = my_dict.get('key', 'new_value')
# 1000000 loops, best of 5: 123 ns per loop

%timeit my_dict.setdefault('key', 'new_value')
# 10000000 loops, best of 5: 90.7 ns per loop

%timeit if 'key' not in my_dict: my_dict['key'] = 'new_value'
# 10000000 loops, best of 5: 51 ns per loop

Check with dis

>>> import dis

>>> my_dict = {}
>>> dis.dis("my_dict.setdefault('key', 'new_value')")

  1           0 LOAD_NAME                0 (my_dict)
              2 LOAD_METHOD              1 (setdefault)
              4 LOAD_CONST               0 ('key')
              6 LOAD_CONST               1 ('new_value')
              8 CALL_METHOD              2
             10 RETURN_VALUE

>>> dis.dis("my_dict['key'] = my_dict.get('key', 'new_value')")

  1           0 LOAD_NAME                0 (my_dict)
              2 LOAD_METHOD              1 (get)
              4 LOAD_CONST               0 ('key')
              6 LOAD_CONST               1 ('new_value')
              8 CALL_METHOD              2
             10 LOAD_NAME                0 (my_dict)
             12 LOAD_CONST               0 ('key')
             14 STORE_SUBSCR
             16 LOAD_CONST               2 (None)
             18 RETURN_VALUE

>>> dis.dis("if 'key' not in my_dict: my_dict['key'] = 'new_value'")

1             0 LOAD_CONST               0 ('key')
              2 LOAD_NAME                0 (my_dict)
              4 COMPARE_OP               7 (not in)
              6 POP_JUMP_IF_FALSE       16
              8 LOAD_CONST               1 ('new_value')
             10 LOAD_NAME                0 (my_dict)
             12 LOAD_CONST               0 ('key')
             14 STORE_SUBSCR
        >>   16 LOAD_CONST               2 (None)
             18 RETURN_VALUE