在 Python 的 For 循环中使用“与”运算符

Question

So I'm a bit curious about why this doesn't work.

How come code like:

for a in range(10) and b in range(10):
  print a + b

generates an error that says 'b is not defined'?

Also, code like:

for a,b in range(10):
  print a + b

generates an error: 'int objects are not iterable'.

Why? I haven't established their value beforehand, so how would Python know they are int objects? Also, I know you could use a while loop instead, but is there any way to carry out the sort of operation I'm doing using a for-loop alone?

Answer 1

for a,b in zip(range(10),range(10)):
    print a + b

should work great... assuming I understood your question properly if not then

for a in range(10):
    for b in range(10):
        print a+b

or even [a+b for a in range(10) for b in range(10)]

Answer 2

Other answers explained the right way to do it, but nobody explained what was wrong with what you did yet.

for a in range(10) and b in range(10):
  print a + b

That's a cute idea of some intuitive syntax, but Python doesn't know that one. The precedence of what you programmed actually works out like this:

for a in ((range(10)) and (b in range(10))):

Python thinks you're trying to make a complex expression to generate a single iterable to iterate over. The first error occurs when it tries to evaluate b to build the value. If b was defined, then b in range(10) would result in True or False . The result of anding it with range(10) will also be a boolean. Then you'd hit another error trying to iterate over a boolean.

for a,b in range(10):
  print a + b

This kind of syntax works, if the enumeration on the right contains elements that are 2-tuples. The first step in this for loop is the equivalent of trying a,b = 0 . It tries to "unpack" the right hand side by iterating over it. But you can't iterate over a single integer. a and b are not defined yet, but the first element of range(10) is. That's the integer you can't iterate over.

Answer 3

You can only iterate over a single iterable at a time in a for loop, the code in your question as it is, is invalid. I believe this is what you intended - iterating over the two ranges simultaneously :

for a, b in zip(range(10), range(10)):
    print a + b

The zip function creates a single iterable of 2-element tuples, taking one element from each range, and then we can unpack each element from the tuple into separate variables.

EDIT:

If you were trying to nest one loop inside the other, the standard way to do it would be this:

for a in range(10):
    for b in range(10):
        print a + b

But we can achieve the same effect by using itertools.product , which looks closer to what you had in mind in the first place:

import itertools as it
for a, b in it.product(range(10), range(10)):
    print a + b