This code below works correct :
def file_gen(f_name):
f = open(f_name)
for line in f:
yield line
gen_line = file_gen("foo.html")
gen_line.next() # '<!DOCTYPE>n'
gen_line.next() # '<html> n'
gen_line.next() # ... next line in file
But this function raises StopIteration. I don’t understand why ?
def file_gen(f_name):
f = open(f_name)
line = f.readline()
yield line
gen_line = file_gen('foo.html')
gen_line.next() # '<!DOCTYPE>n'
gen_line.next() # StopIteration
Advertisement
Answer
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