Discussion on: 3 Common Mistakes that Python Newbies Make

[Thomas Landin]

Oh, and generally working against the language and its idioms by shoehorning things into place instead of picking an appropriate data structure or language construct from the outset.

Examples:

Wanting dicts to be ordered instead of using OrderedDict (this is changed in Python 3.7+).

Looping through dicts and accessing elements by key instead of using the appropriate iterator.

Using for loops and indexing to access parts of lists instead of using slices.

Things like that.

[Idan Arye]

dict ordering is important because keyword arguments are stored in a dict. Consider this:

$ python3.4
Python 3.4.8 (default, Jun 23 2018, 02:35:33) 
[GCC 8.1.1 20180531] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> from collections import OrderedDict
>>> OrderedDict(a=1, b=2, c=3)
OrderedDict([('a', 1), ('c', 3), ('b', 2)])
>>> OrderedDict(c=1, b=2, a=3)
OrderedDict([('a', 3), ('c', 1), ('b', 2)])

We created an OrderedDict, but the constructor was using **kwargs which means the arguments we passed to it were stored in a regular dict and got reordered before OrderedDict.__init__ could store them in order.

With Python 3.6+, the **kwargs stores the arguments in the same order they were written, and the OrderedDict construction works as expected:

$ python3.6
Python 3.6.6 (default, Jun 27 2018, 13:11:40) 
[GCC 8.1.1 20180531] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> from collections import OrderedDict
>>> OrderedDict(a=1, b=2, c=3)
OrderedDict([('a', 1), ('b', 2), ('c', 3)])
>>> OrderedDict(c=1, b=2, a=3)
OrderedDict([('c', 1), ('b', 2), ('a', 3)])

[rhymes]

A little detail on ordered dicts:

Technically dicts have been ordered since 3.6 on (C)Python, it's just that in 3.7 they decided that to be call "Python 3.7" any implementation has to have the insertion order.

So, dicts in Python 3.6+ are ordered only in the official version, which means that you might have portability issues if you depend on that behavior, but in 3.7+ you are garanteed they will be ordered, regardless of the implementation.

Difference with Python 2:

Python 2.7.15 (default, Jul  3 2018, 10:08:31)
[GCC 4.2.1 Compatible Apple LLVM 9.1.0 (clang-902.0.39.2)] on darwin
Type "help", "copyright", "credits" or "license" for more information.
>>> a = {'b': 1, 'c': 3, 'a': '4'}
>>> a
{'a': '4', 'c': 3, 'b': 1}
>>>

Python 3.6.6 (default, Jul 20 2018, 18:34:07)
[GCC 4.2.1 Compatible Apple LLVM 9.1.0 (clang-902.0.39.2)] on darwin
Type "help", "copyright", "credits" or "license" for more information.
>>> a = {'b': 1, 'c': 3, 'a': '4'}
>>> a
{'b': 1, 'c': 3, 'a': '4'}

[Thomas Landin]

Yep, that's what I meant by 3.7 being different on that point.