Why Are Dictionary Keys in Older Python Versions Not Always Ordered?

Order of Keys in Dictionaries in Old Versions of Python

In Python versions prior to 3.6, the order of keys in dictionaries was not guaranteed. This could lead to unexpected results when iterating over a dictionary or accessing its keys.

To understand the issue, consider the following code:

d = {'a': 0, 'b': 1, 'c': 2}
l = d.keys()
print(l)

In this example, the keys() method returns a list of the keys in the dictionary. However, the order of the keys in the list is not guaranteed. In Python versions 2.7 and earlier, the keys were returned in insertion order. However, in Python 3.0 onwards, the keys were returned in arbitrary order.

This inconsistency can be problematic if you rely on the order of the keys in a dictionary. For example, if you use a dictionary to store the items in a grocery list, you may expect the keys to be returned in the order that you added them to the dictionary. However, in Python versions 3.0 onwards, this is not guaranteed.

To解决这个问题，有两种选择。第一个是使用 collections.OrderedDict 类。这个类提供了对插入顺序的保证。以下代码展示了如何使用 OrderedDict：

from collections import OrderedDict

d = OrderedDict([('a', 0), ('b', 1), ('c', 2)])
l = list(d.keys())
print(l)  # ['a', 'b', 'c']

另一个选择是使用 sorted() function to sort the keys in ascending order. However, this requires an additional step and may not always be the desired behavior.

For Python versions 3.6 and later, dictionaries by default maintain insertion order. However, it is still recommended to use OrderedDict if you require a guaranteed insertion order across different implementations of Python.

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