None value returned

Question

The code below is used in a function:

def print_query(x):
    h = open('/home/rv/data.txt', 'r')
    read = h.readlines()
    for line in read:
        return line

When the value "line" is retunred it should print but instead i get the value "None"

Answer 1

You are not checking if the "read" variable actually contains any lines - if it does not, then the function will fall through the for loop and return None.

Using the for loop is also silly - why would you read all lines, and only return the first one, especially in a for loop? What happens when the file can't be opened?

Answer 2

Try this:

with open('/home/rv/data.txt','r') as fh:
    for line in fh:
        print line

If you're on Python 2.5 you might need a from __future__ import with_statement on top.

Also: why do you return the line when you want to print it?

Answer 3

I'm not sure why you're bothering with a loop in your function, since it's just going to return after the first iteration anyway. What exactly are you trying to accomplish? It seems like there is a deeper conceptual issue here, beyond the simple implementation issues.

Answer 4

When you are writing:

def f:
  return 1
  return 2

the function returns 1.

if you want to return few values you can return it in a list:

def f:
  ans = []
  ans.append(1)
  ans.append(2)
  return ans

another option is to use "yield". google it when you will ready for it

Answer 5

Another problem with the function is that you're iterating over the wrong object being more verbose than you need to be. To iterate over lines in a file, just do this:

for line in open(file, "r"):
    print line