Misunderstood python yield

You have:

def file_gen(f_name):
    f = open(f_name)
    line = f.readline()
    yield line

Notice line = f.readline() This only reads 1 line from the file.

Compare:

def g(x):
    li=range(x)
    yield li.pop()

print list(g(10))
# [9]

with this:

def g(x):
    li=range(x)
    while li:
       yield li.pop()

print list(g(10))
# [9, 8, 7, 6, 5, 4, 3, 2, 1, 0]

yield can only be called once with a particular object or expression. Once it is used by the receiver it must be regenerated. So you need a loop around reading each line of the file.

You can use your second (less readable) form this way:

def file_gen(f_name):
    f = open(f_name)
    while True:
        line = f.readline()
        if not line:
            break
        yield line

You need a loop to create the the items to yield. In your first case, for line in f: yield line is a loop.

I would rewrite your function this way:

def file_gen(f_name):
    with open(f_name) as f:
        for line in f:
            yield line

You get StopIteration on the second next() because you've only yielded one result. Did you mean to do this instead?

def file_gen(f_name):
    f = open(f_name)
    lines = f.readlines()
    for line in lines:
        yield line