I have the following code:
import itertools
for c in ((yield from bin(n)[2:]) for n in range(10)):
print(c)
The output is:
0
None
1
None
1
0
None
1
1
None
... etc. Why do the None
s appear? If I instead have:
def hmm():
for n in range(10):
yield from bin(n)[2:]
for c in hmm():
print(c)
Then I get what I would expect:
0
1
1
0
1
1
... etc. Further, is there a way to write it as the generator expression to get the same result as the latter?
yield
is an expression, and its value is whatever is sent into the generator withsend
. If nothing is sent in, the value ofyield
is None. In your exampleyield from
yields the values from the list, but the value of theyield from
expression itself is None, which is yielded at each iteration of the enclosing generator expression (i.e., every value ofrange(10)
).Your example is equivalent to:
Note the extra
yield
.You will always have this issue if you try to use
yield
in a generator expression, because the generator expression already yields its target values, so if you add an explicityield
, you are adding an extra expression (theyield
expression) whose value will also be output in the generator. In other words, something like(x for x in range(5))
already yields the numbers inrange(5)
; if you do something like((yield x) for x in range(5))
, you're going to get the values of theyield
expression in addition to the numbers.As far as I know, there is no way to get the simple
yield from
behavior (without extra Nones) using a generator comprehension. For your case, I think you can achieve what you want by usingitertools.chain.from_iterable
:(Edit: I realized you can get the
yield from
behavior in a generator comprehension by using nestedfor
clauses:x for n in range(10) for x in bin(n)[2:]
. However, I don't think this is any more readable than usingitertools.chain
.)