python-3.xiterationgeneratoryield

Python 3.x: Test if generator has elements remaining


When I use a generator in a for loop, it seems to "know", when there are no more elements yielded. Now, I have to use a generator WITHOUT a for loop, and use next() by hand, to get the next element. My problem is, how do I know, if there are no more elements?

I know only: next() raises an exception (StopIteration), if there is nothing left, BUT isn't an exception a little bit too "heavy" for such a simple problem? Isn't there a method like has_next() or so?

The following lines should make clear, what I mean:

#!/usr/bin/python3

# define a list of some objects
bar = ['abc', 123, None, True, 456.789]

# our primitive generator
def foo(bar):
    for b in bar:
        yield b

# iterate, using the generator above
print('--- TEST A (for loop) ---')
for baz in foo(bar):
    print(baz)
print()

# assign a new iterator to a variable
foobar = foo(bar)

print('--- TEST B (try-except) ---')
while True:
    try:
        print(foobar.__next__())
    except StopIteration:
        break
print()

# assign a new iterator to a variable
foobar = foo(bar)

# display generator members
print('--- GENERATOR MEMBERS ---')
print(', '.join(dir(foobar)))

The output is as follows:

--- TEST A (for loop) ---
abc
123
None
True
456.789

--- TEST B (try-except) ---
abc
123
None
True
456.789

--- GENERATOR MEMBERS ---
__class__, __delattr__, __doc__, __eq__, __format__, __ge__, __getattribute__, __gt__, __hash__, __init__, __iter__, __le__, __lt__, __name__, __ne__, __new__, __next__, __reduce__, __reduce_ex__, __repr__, __setattr__, __sizeof__, __str__, __subclasshook__, close, gi_code, gi_frame, gi_running, send, throw

Thanks to everybody, and have a nice day! :)


Solution

  • The two statements you wrote deal with finding the end of the generator in exactly the same way. The for-loop simply calls .next() until the StopIteration exception is raised and then it terminates.

    http://docs.python.org/tutorial/classes.html#iterators

    As such I don't think waiting for the StopIteration exception is a 'heavy' way to deal with the problem, it's the way that generators are designed to be used.