CODEWITHSUNDEEP
pythongeneratoryield
A.M. Sultanov
A.M. Sultanov

Reputation: 147

Misunderstood python yield

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

Upvotes: 9

Views: 3044

Answers (4)

user14188894
user14188894

Reputation:

strong text

def text_lines(path):
   textFile = open(path, "r")
   while True:
      line = textFile.readline()
      if line :
         yield line.strip()
      else:
         break
   textFile.close()

Upvotes: 0

dawg
dawg

Reputation: 103694

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

Upvotes: 9

R Hyde
R Hyde

Reputation: 10409

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

Upvotes: 2

gonkan
gonkan

Reputation: 259

doesn't

line = f.readline()

only give you one line to yield? therefore iteration stops after that...

Upvotes: 1

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