I am a Python beginner. I find that the else in for-else and while-else is completely unnecessary. Because for and while will finally run to else, and we can use the usual lines instead.
For example:
for i in range(1, 5):
print i
else:
print 'over'
and
for i in range(1, 5):
print i
print 'over'
are the same.
So why does Python have else in for-else and while-else?
You are wrong about the semantics of for/else. The else clause runs only if the loop completed, for example, if a break statement wasn't encountered.
The typical for/else loop looks like this:
for x in seq:
if cond(x):
break
else:
print "Didn't find an x I liked!"
Think of the "else" as pairing with all of the "if's" in the loop body. Your samples are the same, but with "break" statements in the mix, they are not.
A longer description of the same idea: http://nedbatchelder.com/blog/201110/forelse.html
The for ... else statement is used to implement search loops.
In particular, it handles the case where a search loop fails to find anything.
for z in xrange(10):
if z == 5:
# We found what we are looking for
print "we found 5"
break # The else statement will not execute because of the break
else:
# We failed to find what we were looking for
print "we failed to find 5"
z = None
print 'z = ', z
output:
we found 5
z = 5
That search is the same as
z = None
for z in xrange(10):
if 5 == z:
# We found what we are looking for
break
if z == None:
print "we failed to find 5"
else:
print "we found 5"
print 'z = ', z
Remember that for doesn't initialize z if the search list is empty (i.e. []). That's why we have to ensure that z is defined when we use it after the search. The following will raise an exception because z is not defined when we try to print it.
for z in []:
if 5 == z:
break
print "z = ",z
output
print "z = ",z
NameError: name 'z' is not defined
In summary, the else clause will execute whenever the for loop terminates naturally. If a break or an exception occurs in the for loop the else statement will not execute.
