字典，列表或集合末尾的额外逗号是否有任何特殊含义？

Question

I noticed by chance that adding an extra separator comma at the end of a list, dict or set is syntactically correct and does not seem to add anything to the data structure:

In [1]: d1 = {'a': 1, 'b': 2}

In [2]: d2 = {'c': 10, 'd': 20,}

In [3]: d1
Out[3]: {'a': 1, 'b': 2}

In [4]: d2
Out[4]: {'c': 10, 'd': 20}

Does it have any special meaning or usage? The only one I found is to explicit a data structure during an initialization:

In [14]: r = (1)

In [15]: r
Out[15]: 1

In [16]: r = (1,)

In [17]: r
Out[17]: (1,)

Answer 1

It has no special meaning in a list or dictionary, but can be useful when using source code change management tools, see below.

Non-empty tuples are defined by using a comma between elements, the parentheses are optional and only required in contexts where the comma could have a different meaning.

Because the comma defines the tuple, you need at least one comma if there is just the one element:

>>> 1
1
>>> 1,
(1,)
>>> type((1,)) # need parens to distinguish comma from the argument separator
<type 'tuple'>

The empty tuple is defined by using empty parentheses:

>>> type(())
<type 'tuple'>

The trailing comma can be helpful in minimising how many lines changed when adding new lines; adding an additional line to a dictionary with a trailing comma would not change the last existing entry:

a_value = {
    key1: value1,
    key2: value2,
    # inserting here doesn't require adding a comma
    # to the preceding line.
}

Answer 2

It's useful for consistency across multiple lines:

d1 = {
    'a': 1,
    'b': 2,
}

You can rearrange or add lines without changing commas. It doesn't change anything about the information represented.

Answer 3

You can use dis module to verify that behaviour of both objects is exactly the same.

In [10]: import dis

In [11]: def make_dict():
    return {"a": 1, "b": 2}
   ....: 

In [12]: def make_dict_2():
    return {"a": 1, "b": 2,}
   ....: 

In [13]: dis.dis(make_dict)
  2           0 BUILD_MAP                2
              3 LOAD_CONST               1 (1)
              6 LOAD_CONST               2 ('a')
              9 STORE_MAP           
             10 LOAD_CONST               3 (2)
             13 LOAD_CONST               4 ('b')
             16 STORE_MAP           
             17 RETURN_VALUE        

In [14]: dis.dis(make_dict_2)
  2           0 BUILD_MAP                2
              3 LOAD_CONST               1 (1)
              6 LOAD_CONST               2 ('a')
              9 STORE_MAP           
             10 LOAD_CONST               3 (2)
             13 LOAD_CONST               4 ('b')
             16 STORE_MAP           
             17 RETURN_VALUE