Python空生成器函数

In python, one can easily define an iterator function, by putting the yield keyword in the function’s body, such as:

def gen():
    for i in range(100):
        yield i

How can I define a generator function that yields no value (generates 0 values), the following code doesn’t work, since python cannot know that it is supposed to be an generator and not a normal function:

def empty():
    pass

I could do something like

def empty():
    if False:
        yield None

But that would be very ugly. Is there any nice way to realize an empty iterator function?

You can use return once in a generator; it stops iteration without yielding anything, and thus provides an explicit alternative to letting the function run out of scope. So use yield to turn the function into a generator, but precede it with return to terminate the generator before yielding anything.

>>> def f():
...     return
...     yield
... 
>>> list(f())
[]

I’m not sure it’s that much better than what you have — it just replaces a no-op if statement with a no-op yield statement. But it is more idiomatic. Note that just using yield doesn’t work.

>>> def f():
...     yield
... 
>>> list(f())
[None]

Why not just use iter(())?

This question asks specifically about an empty generator function. For that reason, I take it to be a question about the internal consistency of Python’s syntax, rather than a question about the best way to create an empty iterator in general.

If question is actually about the best way to create an empty iterator, then you might agree with Zectbumo about using iter(()) instead. However, it’s important to observe that iter(()) doesn’t return a function! It directly returns an empty iterable. Suppose you’re working with an API that expects a callable that returns an iterable each time it’s called, just like an ordinary generator function. You’ll have to do something like this:

def empty():
    return iter(())

(Credit should go to Unutbu for giving the first correct version of this answer.)

Now, you may find the above clearer, but I can imagine situations in which it would be less clear. Consider this example of a long list of (contrived) generator function definitions:

def zeros():
    while True:
        yield 0

def ones():
    while True:
        yield 1

...

At the end of that long list, I’d rather see something with a yield in it, like this:

def empty():
    return
    yield

or, in Python 3.3 and above (as suggested by DSM), this:

def empty():
    yield from ()

The presence of the yield keyword makes it clear at the briefest glance that this is just another generator function, exactly like all the others. It takes a bit more time to see that the iter(()) version is doing the same thing.

It’s a subtle difference, but I honestly think the yield-based functions are more readable and maintainable.

See also this great answer from user3840170 that uses dis to show another reason why this approach is preferable: it emits the fewest instructions when compiled.

iter(())

You don’t require a generator. C’mon guys!

Python 3.3 (because I’m on a yield from kick, and because @senderle stole my first thought):

>>> def f():
...     yield from ()
... 
>>> list(f())
[]

But I have to admit, I’m having a hard time coming up with a use case for this for which iter([]) or (x)range(0) wouldn’t work equally well.

Another option is:

(_ for _ in ())

Must it be a generator function? If not, how about

def f():
    return iter(())

The “standard” way to make an empty iterator appears to be iter([]). I suggested to make [] the default argument to iter(); this was rejected with good arguments, see http://bugs.python.org/issue25215 – Jurjen

Like @senderle said, use this:

def empty():
    return
    yield

I’m writing this answer mostly to share another justification for it.

One reason for choosing this solution above the others is that it is optimal as far as the interpreter is concerned.

>>> import dis
>>> def empty_yield_from():
...     yield from ()
... 
>>> def empty_iter():
...     return iter(())
... 
>>> def empty_return():
...     return
...     yield
...
>>> def noop():
...     pass
...
>>> dis.dis(empty_yield_from)
  2           0 LOAD_CONST               1 (())
              2 GET_YIELD_FROM_ITER
              4 LOAD_CONST               0 (None)
              6 YIELD_FROM
              8 POP_TOP
             10 LOAD_CONST               0 (None)
             12 RETURN_VALUE
>>> dis.dis(empty_iter)
  2           0 LOAD_GLOBAL              0 (iter)
              2 LOAD_CONST               1 (())
              4 CALL_FUNCTION            1
              6 RETURN_VALUE
>>> dis.dis(empty_return)
  2           0 LOAD_CONST               0 (None)
              2 RETURN_VALUE
>>> dis.dis(noop)
  2           0 LOAD_CONST               0 (None)
              2 RETURN_VALUE

As we can see, the empty_return has exactly the same bytecode as a regular empty function; the rest perform a number of other operations that don’t change the behaviour anyway. The only difference between empty_return and noop is that the former has the generator flag set:

>>> dis.show_code(noop)
Name:              noop
Filename:          <stdin>
Argument count:    0
Positional-only arguments: 0
Kw-only arguments: 0
Number of locals:  0
Stack size:        1
Flags:             OPTIMIZED, NEWLOCALS, NOFREE
Constants:
   0: None
>>> dis.show_code(empty_return)
Name:              empty_return
Filename:          <stdin>
Argument count:    0
Positional-only arguments: 0
Kw-only arguments: 0
Number of locals:  0
Stack size:        1
Flags:             OPTIMIZED, NEWLOCALS, GENERATOR, NOFREE
Constants:
   0: None

Of course, the strength of this argument is very dependent on the particular implementation of Python in use; a sufficiently smart alternative interpreter may notice that the other operations amount to nothing useful and optimise them out. However, even if such optimisations are present, they require the interpreter to spend time performing them and to safeguard against optimisation assumptions being broken, like the iter identifier at global scope being rebound to something else (even though that would most likely indicate a bug if it actually happened). In the case of empty_return there is simply nothing to optimise, so even the relatively naïve CPython will not waste time on any spurious operations.

generator = (item for item in [])