Python 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
       }
         


        

Answer 1

You can test if a number of keys are in a dict by taking advantage that .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 .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():
    ...