More Dictionary Methods

Dictionary methods provide a variety of tools. For instance, the dictionary values and items methods return the dictionary’s values and (key,value) pair tuples, respectively (as with keys, wrap them in a list call in Python 3.0 to collect their values for display):

>>> D = {'spam': 2, 'ham': 1, 'eggs': 3}
>>> list(D.values())
[3, 1, 2]
>>> list(D.items())
 [('eggs', 3), ('ham', 1), ('spam', 2)]

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Such lists are useful in loops that need to step through dictionary entries one by one. Fetching a nonexistent key is normally an error, but the get method returns a default value (None, or a passed-in default) if the key doesn’t exist. It’s an easy way to fill in a default for a key that isn’t present and avoid a missing-key error:

>>> D.get('spam')                          # A key that is there
2
>>> print(D.get('toast'))                  # A key that is missing
None
>>> D.get('toast', 88)
88

The update method provides something similar to concatenation for dictionaries, though it has nothing to do with left-to-right ordering (again, there is no such thing in dictionaries). It merges the keys and values of one dictionary into another, blindly overwriting values of the same key:

>>> D
{'eggs': 3, 'ham': 1, 'spam': 2}
>>> D2 = {'toast':4, 'muffin':5}
>>> D.update(D2)
>>> D
{'toast': 4, 'muffin': 5, 'eggs': 3, 'ham': 1, 'spam': 2}

Finally, the dictionary pop method deletes a key from a dictionary and returns the value it had. It’s similar to the list pop method, but it takes a key instead of an optional position:

# pop a dictionary by key
>>> D
{'toast': 4, 'muffin': 5, 'eggs': 3, 'ham': 1, 'spam': 2}
>>> D.pop('muffin')
5
>>> D.pop('toast')                         # Delete and return from a key
4
>>> D
{'eggs': 3, 'ham': 1, 'spam': 2}

# pop a list by position
>>> L = ['aa', 'bb', 'cc', 'dd']
>>> L.pop()                                # Delete and return from the end
'dd'
>>> L
['aa', 'bb', 'cc']
>>> L.pop(1)                               # Delete from a specific position
'bb'
>>> L
['aa', 'cc']

Dictionaries also provide a copy method; we’ll discuss this in Chapter 9, as it’s a way to avoid the potential side effects of shared references to the same dictionary. In fact, dictionaries come with many more methods than those listed in Table 8-2; see the Python library manual or other documentation sources for a comprehensive list.