Python's "in" set operator


I’m a little confused about the python in operator for sets.

If I have a set s and some instance b, is it true that b in s means “is there some element x in s such that b == x is true“?

Asked By: Dejas



Yes, but it also means hash(b) == hash(x), so equality of the items isn’t enough to make them the same.

That’s right. You could try it in the interpreter like this:

>>> a_set = set(['a', 'b', 'c'])

>>> 'a' in a_set

>>>'d' in a_set
Answered By: Niclas Nilsson

Strings, though they are not set types, have a valuable in property during validation in scripts:

yn = input("Are you sure you want to do this? ")
if yn in "yes":
    #accepts 'y' OR 'e' OR 's' OR 'ye' OR 'es' OR 'yes'
    return True
return False

I hope this helps you better understand the use of in with this example.

Answered By: yurisich

Yes it can mean so, or it can be a simple iterator. For example:
Example as iterator:

for x in a:
 print ('This set contains the value ' + x)

Similarly as a check:

if 'I' in a:
 print ('There is an "I" in here')

edited: edited to include sets rather than lists and strings

Answered By: Arnab Ghosal

Sets behave different than dicts, you need to use set operations like issubset():

>>> k
{'ip': '', 'pw': 'test1234', 'port': 1234, 'debug': True}
>>> set('ip,port,pw'.split(',')).issubset(set(k.keys()))
>>> set('ip,port,pw'.split(',')) in set(k.keys())
Answered By: MortenB

List’s __contains__ method uses the __eq__ method of its elements. Whereas set’s __contains__ uses __hash__. Have a look to the following example that I wish will be explicit:

class Salary:
    """An employee receives one salary for each job he has."""

    def __init__(self, value, job, employee):
        self.value = value
        self.job = job
        self.employee = employee

    def __repr__(self):
        return f"{self.employee} works as {self.job} and earns {self.value}"

    def __eq__(self, other):
        """A salary is equal to another if value is equal."""
        return self.value == other.value

    def __hash__(self):
        """A salary can be identified with the couple employee-job."""
        return hash(self.employee) + hash(self.job)

alice = 'Alice'
bob = 'Bob'
engineer = 'engineer'
teacher = 'teacher'

alice_engineer = Salary(10, engineer, alice)
alice_teacher = Salary(8, teacher, alice)
bob_engineer = Salary(10, engineer, bob)

print(alice_engineer == alice_teacher)
print(alice_engineer == bob_engineer, 'n')

print(alice_engineer is alice_engineer)
print(alice_engineer is alice_teacher)
print(alice_engineer is bob_engineer, 'n')

alice_jobs = set([alice_engineer, alice_teacher])
print(bob_engineer in alice_jobs)  # IMPORTANT
print(bob_engineer in list(alice_jobs))  # IMPORTANT

Console prints:



{Alice works as teacher and earns 8, Alice works as engineer and earns 10}
Answered By: Valentin Fabianski
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