How does the max function work when items in a list are not of the same type?
For example the following code returns [1,'3']
max([1,52,53],[1,'3']) => [1,'3']
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3 Answers
In Python2, the default comparison for objects of different types is to compare using the id of their types (obtained by casting the object pointers to integers). Here a link to the source: http://hg.python.org/cpython/file/2.7/Objects/object.c#l757
On my build, here is the ordering of types:
>>> sorted([bool, int, float, long, list, tuple, dict, str, unicode])
[<type 'bool'>, <type 'float'>, <type 'int'>, <type 'list'>, <type 'long'>,
<type 'dict'>, <type 'str'>, <type 'tuple'>, <type 'unicode'>]
Numbers (except for complex) have compare methods that allow cross type comparison based on numeric value (i.e. a float can be compared with an int).
The None object is special. It compares less than everything else.
To see it all put together, use sorted to see the ordering:
>>> sorted(zoo)
[None, -5, -5.0, 0, 0.0, -0.0, False, True, 10, 10.0, 11.5, {},
{'abc': 10}, {'lmno': 20}, [], [1, 2], [1, 2, 3], [1, [2, 3]],
'', u'', 'alpha', u'alpha', 'bingo', 'cat', (), (1, 2),
(1, 2, 3), (1, (2, 3)), u'bingo', u'cat']
answered Apr 19, 2012 at 11:55
Raymond Hettinger
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3 Comments
Sagar Patel
Thanks! But comparing: [1]<(1) returns false. Any idea why?
Ben Hoyt
@Sagar, note that
(1) is actually an int. According to Python syntax, you have to have a comma to distinguish it as a tuple, so try [1] < (1,). Still, list and tuple are objects of different type, so presumably they'll be compared using the id of their types.
hynekcer
Note that
sorted(zoo) looks unsorted because sorting of different types is ambiguous: str < tuple < unicode < str. Example: >>> '' < () < u'' < '' Result: TrueIn Python 2 objects of different types are compared by type's string representation using special logic. See Raymond's answer for details.
In Python 3 this code will raise an exception:
Traceback (most recent call last):
File "prog.py", line 1, in <module>
max([1,52,53],[1,'3'])
TypeError: unorderable types: str() > int()
3 Comments
Raymond Hettinger
Sorry, this isn't correct. The default comparison does not use the type's string representation.
Sagar Patel
Could you please elaborate what is "type's string representation"?
hynekcer
Not exactly by name but similarly obsure.
It does whatever the > operator does for the two elements.
