Python 3.7 以降で辞書に順次インデックスを付ける方法

Indexing into a Dictionary: A Python 3.7 and Later Approach

In a dictionary, accessing values by key is the typical operation. However, in some cases, it may be desirable to index into a dictionary's entries sequentially as if it were a list. However, unlike lists, dictionaries do not have an inherent ordering.

Pre-Python 3.7 Dictionaries

In Python versions prior to 3.7, dictionaries were not ordered, and accessing the first entry using an index like colors[0] would result in a KeyError.

Ordered Dictionaries in Python 3.7 and Later

Starting with Python 3.7, dictionaries have become order-preserving, meaning they maintain the order of insertion. This behavior is similar to an OrderedDict from the collections module.

Accessing the First Entry

Despite the new ordering, there is still no dedicated method for indexing into a dictionary's keys or values directly. However, the following approaches can be used:

• Convert to a List: Create a list of either the keys or values and access the first item like this:

<code class="python">first_key = list(colors)[0]
first_val = list(colors.values())[0]</code>

• Use a Custom Function: Define a helper function to iterate over the keys, returning the desired entry based on its index:

<code class="python">def get_first_key(dictionary):
    for key in dictionary:
        return key
    raise IndexError

first_key = get_first_key(colors)
first_val = colors[first_key]</code>

Accessing the nth Entry

For indexing into keys or values beyond the first, a similar custom function approach can be used:

<code class="python">def get_nth_key(dictionary, n=0):
    if n < 0:
        n += len(dictionary)
    for i, key in enumerate(dictionary.keys()):
        if i == n:
            return key
    raise IndexError("dictionary index out of range") </code>

By using these approaches, you can index into dictionaries in a manner similar to lists, even though dictionaries are not inherently ordered.

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