pprint sorting dicts but not sets?
Question:
I know that dicts and sets aren’t ordered, so equal sets or dicts may print differently (all tests with Python 3.6.1):
>>> for obj in {0, 8}, {8, 0}, {0:0, 8:8}, {8:8, 0:0}:
print(obj)
{0, 8}
{8, 0}
{0: 0, 8: 8}
{8: 8, 0: 0}
And I just realized that pprint
(“pretty-print”) sorts dicts but not sets:
>>> for obj in {0, 8}, {8, 0}, {0:0, 8:8}, {8:8, 0:0}:
pprint.pprint(obj)
{0, 8}
{8, 0}
{0: 0, 8: 8}
{0: 0, 8: 8}
It’s documentation also says “Dictionaries are sorted by key before the display is computed”. But why doesn’t it also sort sets? Doesn’t seem pretty to me. And is there a way to make it sort sets? Also inside nested structures, as that’s a main purpose of pprint
.
Answers:
This was raised in issue 27495 and it is a bug, rather than just a design choice, but apparently has not yet been resolved.
Here is another example from the issue that illustrates perhaps more obviously the behavior you identify in Python 3:
>>> import string, pprint
>>> pprint.pprint(set(string.digits))
{'7', '1', '9', '8', '3', '0', '2', '5', '6', '4'}
The same applies for frozenset()
too, but note that multi-line pprint
outputs are sorted in Python 3, for example:
>>> pprint.pprint(set(string.digits), width=1)
{'0',
'1',
'2',
'3',
'4',
'5',
'6',
'7',
'8',
'9'}
However, in Python 2, the output from the same original code is sorted:
>>> pprint.pprint(set(string.digits))
set(['0', '1', '2', '3', '4', '5', '6', '7', '8', '9'])
I think it is the inconsistency between Python 3 and Python 2, and between the single-line multi-line behavior, that makes this a bug.
For dict
s, a similar example, illustrates as you note, that the output is sorted in either Python 3 or 2, as it should be:
>>> pprint.pprint({i:None for i in set(string.digits)})
{'0': None,
'1': None,
'2': None,
'3': None,
'4': None,
'5': None,
'6': None,
'7': None,
'8': None,
'9': None}
However, for Python 3.6, it could be considered surprising that pprint
sorts dict
s since they are ordered now. However, since this is just an implementation detail (for now) I guess there is no obligation for pprint
to maintain the insertion order (yet), and doing so would break pprint
‘s own consistency across Python versions of always sorting dict
s.
I know that dicts and sets aren’t ordered, so equal sets or dicts may print differently (all tests with Python 3.6.1):
>>> for obj in {0, 8}, {8, 0}, {0:0, 8:8}, {8:8, 0:0}:
print(obj)
{0, 8}
{8, 0}
{0: 0, 8: 8}
{8: 8, 0: 0}
And I just realized that pprint
(“pretty-print”) sorts dicts but not sets:
>>> for obj in {0, 8}, {8, 0}, {0:0, 8:8}, {8:8, 0:0}:
pprint.pprint(obj)
{0, 8}
{8, 0}
{0: 0, 8: 8}
{0: 0, 8: 8}
It’s documentation also says “Dictionaries are sorted by key before the display is computed”. But why doesn’t it also sort sets? Doesn’t seem pretty to me. And is there a way to make it sort sets? Also inside nested structures, as that’s a main purpose of pprint
.
This was raised in issue 27495 and it is a bug, rather than just a design choice, but apparently has not yet been resolved.
Here is another example from the issue that illustrates perhaps more obviously the behavior you identify in Python 3:
>>> import string, pprint >>> pprint.pprint(set(string.digits)) {'7', '1', '9', '8', '3', '0', '2', '5', '6', '4'}
The same applies for frozenset()
too, but note that multi-line pprint
outputs are sorted in Python 3, for example:
>>> pprint.pprint(set(string.digits), width=1) {'0', '1', '2', '3', '4', '5', '6', '7', '8', '9'}
However, in Python 2, the output from the same original code is sorted:
>>> pprint.pprint(set(string.digits)) set(['0', '1', '2', '3', '4', '5', '6', '7', '8', '9'])
I think it is the inconsistency between Python 3 and Python 2, and between the single-line multi-line behavior, that makes this a bug.
For dict
s, a similar example, illustrates as you note, that the output is sorted in either Python 3 or 2, as it should be:
>>> pprint.pprint({i:None for i in set(string.digits)})
{'0': None,
'1': None,
'2': None,
'3': None,
'4': None,
'5': None,
'6': None,
'7': None,
'8': None,
'9': None}
However, for Python 3.6, it could be considered surprising that pprint
sorts dict
s since they are ordered now. However, since this is just an implementation detail (for now) I guess there is no obligation for pprint
to maintain the insertion order (yet), and doing so would break pprint
‘s own consistency across Python versions of always sorting dict
s.