I have a file that has the following contents:
a
b
c
d
I'm reading this in Python using the following code:
f = open('foo.txt')
w = f.readline()
while w is not '' :
print w
w = f.readline()
According to the docs
if f.readline() returns an empty string, the end of the file has been reached
Why then do I go into an infinite loop?
The is operator tests if two references reference exactly the same object. In this case, you want an equality check as you noted.
The infinite loop happened because although it returned '', it was a different instance of str. Different instance, but same value.
Python may or may not choose as an optimization to resuse an existing str object (they are immutable, after all). You should not depend on this type of behavior in general, however, unless the documentation specifically mentions otherwise (like for None as it's the one and only instance of NoneType).
This is interesting.
If you replace
while w is ''
with
while w != ''
the code would run successfully.
This means that readline() returns a new instance of the empty string, instead of returning the global one which you get when you execute
s = ''
t = ''
assert s is t
Try this
f = open('foo.txt')
while 1:
line = f.readline()
if not line:
break
print line
with open('workfile', 'r') as f:
read_data = f.read()
f.closed
It is good practice to use the with keyword when dealing with file objects. This has the advantage that the file is properly closed after its suite finishes, even if an exception is raised on the way. It is also much shorter than writing equivalent try-finally blocks.
Refer to this link: http://docs.python.org/2/tutorial/inputoutput.html
w != ''makes it work fine. I'd still like to know whyis notcauses an infinite loop.while w is not '' :, just writewhile w: