Check if list of keys exist in dictionary

Question

I have a dictionary that looks like that:

grades = {
        'alex' : 11,
        'bob'  : 10,
        'john' : 14,
        'peter': 7
       }

and a list of names students = ('alex', 'john')

I need to check that all the names in students exist as keys in grades dict.

grades can have more names, but all the names in students should be in grades

There must be a straightforward way to do it, but i’m still new to python and can’t figure it out. tried if students in grades, didn’t work.

In the actual cases, the lists will be much bigger.

Asked By: applechief

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Source

Answer 1

Use all():

if all(name in grades for name in students):
    # whatever

Answered By: Sven Marnach

Answer 2

>>> grades = {
        'alex' : 11,
        'bob'  : 10,
        'john' : 14,
        'peter': 7
}
>>> names = ('alex', 'john')
>>> set(names).issubset(grades)
True
>>> names = ('ben', 'tom')
>>> set(names).issubset(grades)
False

Calling it class is invalid so I changed it to names.

Answered By: jamylak

Answer 3

Assuming students as set

if not (students - grades.keys()):
    print("All keys exist")

If not convert it to set

if not (set(students) - grades.keys()):
    print("All keys exist")

Answered By: abhilekh

Answer 4

You can test if a number of keys are in a dict by taking advantage that <dict>.keys() returns a set.

This logic in code…

if 'foo' in d and 'bar' in d and 'baz' in d:
    do_something()

can be represented more briefly as:

if {'foo', 'bar', 'baz'} <= d.keys():
    do_something()

The <= operator for sets tests for whether the set on the left is a subset of the set on the right. Another way of writing this would be <set>.issubset(other).

There are other interesting operations supported by sets: https://docs.python.org/3.8/library/stdtypes.html#set

Using this trick can condense a lot of places in code that check for several keys as shown in the first example above.

Whole lists of keys could also be checked for using <=:

if set(students) <= grades.keys():
    print("All studends listed have grades in your class.")

# or using unpacking - which is actually faster than using set()
if {*students} <= grades.keys():
    ...

Or if students is also a dict:

if students.keys() <= grades.keys():
    ...

Answered By: Todd

Check if list of keys exist in dictionary

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