## TIL #069 β generator membership testing

Today I learned that generators support membership testing with the operator in.

# Generator membership testing

Membership testing in Python boils down to using the operator in, as the REPL session excerpt shows:

>>> my_list = [42, 73, 16, 0, 10]
>>> 42 in my_list
True
>>> 43 in my_list
False
>>> 10 in my_list
True

What I hadn't realised yet is that generators also support membership testing! It is very easy to verify that they do support membership testing, though:

def generator():
yield 42
yield 73
yield 16
yield 0
yield 10

gen = generator()
print(42 in gen)  # True

# Gotchas of generator membership testing

What I immediately realised afterwards is that you need to be very careful about membership testing with generators. For example, consider the generator function from before:

def generator():
yield 42
yield 73
yield 16
yield 0
yield 10

We can check if the number 999 is in there and we will get the correct answer:

>>> gen = generator()
>>> 999 in gen
False

But now, think about it. How can Python tell that 999 is not inside the generator gen? It had to iterate over it, which means it had to go through all the yield expressions already! In turn, this means that the generator was already exhausted. It is already empty:

>>> next(gen)
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
StopIteration

In practice, if a generator reports that a given value is not in the generator, then it is probably because of this issue; because you had already iterated past the value that you were looking for.

Let me show you another example of this:

>>> def generator():
...     yield 1
...     yield 2
...     yield 3
...
>>> gen = generator()
>>> 999 in gen
False
>>> 1 in gen
False
>>> 2 in gen
False
>>> 3 in gen
False

This example might look obvious to you because I am using a non-existing value to exhaust the generator. However, this issue can also arise after successful membership tests:

>>> def generator():
...     yield 1
...     yield 2
...     yield 3
...
>>> gen = generator()
>>> 2 in gen
True
>>> 1 in gen
False

To make sure you understand what's going on, try to guess the output of the three membership checks below. Other than that small challenge, that's it for now! Stay tuned and I'll see you around!

>>> def generator():
...     yield 1
...     yield 2
...     yield 3
...
>>> gen = generator()
>>> 3 in gen
# ???
>>> 1 in gen
# ???
>>> 2 in gen
# ???

That's it for now! Stay tuned and I'll see you around!

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