Python：在elif条件下循环

Question

I'm running Python 3.

Is it possible to put a loop in the condition for elif ? Here's the basic idea I'm trying to achieve. The number of items in the list is not constant.

if some_condition:
    do this
elif [ x.method() for x in list ]:
    do this to x
else:
    do something else

Now, this comes to my mind:

if some_condition:
    do this
for x in list:
    if x.method():
        do this to x
        break

But I'm trying to avoid running all the if statements, there's a lot of stuff going on in them. And I would like to get it in the elif part specifically and not in else .

Edit / more clarification:

It seems what I would need is any( x.method() for x in list ) but also with a reference to x so that I can then use x if the condition was true.

Here's the whole concept I'm trying to get again:

if condition:
    do this
elif list[0].method():
    do this to list[0]
elif list[1].method():
    do this to list[1]
...
elif list[n].method():
    do this to list[n]
else:
    do this

where method() is some method that returns True or False , and n is the size of the list and not a constant.

Answer 1

if [ x.method() for x in list ]:

You ask, is it possible? Yes. What does it do, though? Probably not what you intend.

For the number of items in list , it will fabricate one value in the resulting list. For list values, only a list of zero length will be False. [False, False, False] is True, so the body of the condition will be executed.

You may be looking for all() or any() if x.method() is supposed to influence the following condition decision.

if all(x.method() for x in list):   # will require that every *x.method* call return truth.

Likewise, you may want

if any(x.method() for x in list):   # only one required. May not do every .method if it can stop early. Beware side-effects.

Answer 2

I don't think what you want -- to have it entirely in the elif -- is possible. You'd have to evaluate whether there is any such value in the list, and then bind it to x in the condition . As far as I know, this is not possible in Python's syntax. You can not do an assignment in the condition , and while the loop variable in a list comprehension can "leak" to the outside scope, the same is not true for a generator.

>>> if any(x for x in range(10) if x >= 5):
...     print x
NameError: name 'x' is not defined
>>> if any([x for x in range(10) if x >= 5]):
...     print x
9

In the second case (list), we have a reference to x , but it is the last value from the entire list, and in the first case (generator), x can not be resolved at all.

---

Instead, here's another variant, using a generator expression to combine the for with the if , and adding an else to the for to enumate your final else clause.

if some_condition:
    print "do this"
else:
    for x in (x for x in lst if foo(x)):
        print "do this to", x
        break
    else:
        print "do something else"

Answer 3

I see now. The Pythonic way of doing what your updated edit says is to make a for loop and break out after you find your first item and act on it, and use the almost unknown and unused "else" of a loop.

Since the "for / else" idiom is so rarely used, you will need to add a comment about it, for future readers.

if some_test:
    first_action()
else:
    for item in list:
        if item.method():
            do_this(item)
            break   # process no more of list
    else:   # for-else only entered if no break from the list!
        final_action()

Read up on loop elses in the Python Tutorial .

Answer 4

It is not exactly possible to do what you want, but you can do something similar with list comprehensions:

if some_condition:
    do this
else:
    res = [x for x in list if x.method()]
    if res:
        do something to res[0]
    else:
        do something else

To avoid do x.method() for every value, you could also use itertools.dropwhile . The following approach will keep checking x.method() until it finds one that is true, then stop and return just that value of x . If it doesn't find any that are true, it will do something else.

from itertools import dropwhile
if some_condition:
    do this
else:
    try:
        res = next(dropwhile(lambda x: not x.method(), x))
        do something to res
    except StopIteration:
        do something else

Answer 5

This could work as long as you don't need a reference to x outside of the elif block. This feels closest to the syntax you are asking for, but I would still probably go with either Kevin or Tobias's answers as they are more explicit and readable.

 def check_and_do(x):
    b = x.method()
    if b:
        do this to x
    return b

if condition:
    do this
elif any(check_and_do(x) for x in list):
    pass  # Already did this to x in check_and_do
else:
    do something else