Python Language Common Pitfalls Dictionaries are unordered


You might expect a Python dictionary to be sorted by keys like, for example, a C++ std::map, but this is not the case:

myDict = {'first': 1, 'second': 2, 'third': 3}
# Out: {'first': 1, 'second': 2, 'third': 3}

print([k for k in myDict])
# Out: ['second', 'third', 'first']

Python doesn't have any built-in class that automatically sorts its elements by key.

However, if sorting is not a must, and you just want your dictionary to remember the order of insertion of its key/value pairs, you can use collections.OrderedDict:

from collections import OrderedDict

oDict = OrderedDict([('first', 1), ('second', 2), ('third', 3)])

print([k for k in oDict])
# Out: ['first', 'second', 'third']

Keep in mind that initializing an OrderedDict with a standard dictionary won't sort in any way the dictionary for you. All that this structure does is to preserve the order of key insertion.

The implementation of dictionaries was changed in Python 3.6 to improve their memory consumption. A side effect of this new implementation is that it also preserves the order of keyword arguments passed to a function:

Python 3.x3.6
def func(**kw): print(kw.keys())

func(a=1, b=2, c=3, d=4, e=5) 
dict_keys(['a', 'b', 'c', 'd', 'e']) # expected order 

Caveat: beware that the order-preserving aspect of this new implementation is considered an implementation detail and should not be relied upon, as it may change in the future.