Python: What kind of syntax is it to use for after a statement in side a function?

Question

I saw a line of code to generate random string.

''.join(random.choice(string.ascii_uppercase + string.digits) for _ in range(N))

I am not sure what type of usage is this and I cannot find it in python doc.

I try

random.choice(string.ascii_uppercase + string.digits) for _ in range(N)

but it says there's a syntax error

Your syntax error can be easily solved by turning it into either a list comprehension (wrap it in `[]`) or a generator (wrap in `()`). — Jon Surrell, Dec 17 '15 at 09:54
Also related: [list comprehension without \[ \], Python](http://stackoverflow.com/q/9060653) — Martijn Pieters, Dec 17 '15 at 10:00

score 1 · Answer 1 · answered Dec 17 '15 at 09:48

1

It's a generator expression:

The semantics of a generator expression are equivalent to creating an anonymous generator function and calling it. For example:
g = (x**2 for x in range(10))
print g.next()
is equivalent to:
def __gen(exp):
    for x in exp:
        yield x**2
g = __gen(iter(range(10)))
print g.next()

answered Dec 17 '15 at 09:48

jtbandes

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And, notably, *generators* are *iterable*, like lists. So you can call [`str.join`](https://docs.python.org/2/library/stdtypes.html#str.join) on them just like on a list. – Jon Surrell Dec 17 '15 at 09:52

Python: What kind of syntax is it to use for after a statement in side a function?

1 Answers1