CODEWITHSUNDEEP
pythonpython-3.xdictionary
sooh99
sooh99

Reputation: 57

Checking whether a key or value exists in a dictionary

First of all, thank you for your help in advance. I am novice to coding and currently studying Python using the book, 'Automate the Boring Stuff with Python' and I have a conceptual question on multiple assignment. https://automatetheboringstuff.com/chapter5/

Here are the excerpts from the book:

>>> spam = {'name': 'Zophie', 'age': 7}
>>> 'name' in spam.keys()
True
>>> 'Zophie' in spam.values()
True
>>> 'color' in spam.keys()
False
>>> 'color' not in spam.keys()
True
>>> 'color' in spam
False

So I tried,

>>> 7 in spam

expecting True, but got False.

Then I tried,

>>> 7 in spam.values()

which returned True.

So, it seems string values work the same way, but integers behave differently.

Why is this the case?

Upvotes: 2

Views: 119

Answers (2)

albert
albert

Reputation: 8583

In checks against the keys only, whereas the values are accessible via dict.values(), dict.values() gives access to the values of a dictionary. Both .keys()and .values() return an iterable.

For a more elegant/shorter way to check for an existing key you could use dict.get(key). This returns the value of the given key if the key is in the dictionary, and a 'fall-back' return value if not. This 'fall-back' return value is None by default (if not specified) or can be set by the programmer by using dict.get(key, return_if_not_in_dict). For a more detailed explanation see this question or the following code:

In [10]: spam = {'name': 'Zophie', 'age': 7}

In [11]: 'name' in spam
Out[11]: True

In [12]: 'Zophie' in spam
Out[12]: False

In [13]: 'name' in spam.keys()
Out[13]: True

In [14]: 'Zophie' in spam.values()
Out[14]: True

In [15]: type(spam.keys())
Out[15]: dict_keys

In [17]: type(spam.values())
Out[17]: dict_values

In [18]: spam.get('name')
Out[18]: 'Zophie'

In [19]: spam.get('some invalid key')   # returns `None`, so no output here

In [20]: spam.get('some invalid key', 'not in here')
Out[20]: 'not in here'

In [21]: bool(spam.get('name'))
Out[21]: True

In [22]: bool(spam.get('some invalid key'))
Out[22]: False

Upvotes: 0

Agapito Gallart Bernat
Agapito Gallart Bernat

Reputation: 363

Your problem is that 7 is a value, not a key. spam = {'name': 'Zophie', 'age':7} So if you do:

7 in spam >> False
'age' in spam >> True
7 in spam.values() >> True
7 in spam.keys() >> False

foo = {key0 : value0 , key1 : value1 ... keyN: valueN}

Take in to account, that in python the in looks for dictionary's keys list, not the values list. Unless you specify to do so.

Upvotes: 2

Related Questions