How to define an empty generator function?
PythonGeneratorPython Problem Overview
A generator function can be defined by putting the yield
keyword in the function’s body:
def gen():
for i in range(10):
yield i
How to define an empty generator function?
The following code doesn’t work, since Python cannot know that it is supposed to be a generator function instead of a normal function:
def empty():
pass
I could do something like this:
def empty():
if False:
yield
But that would be very ugly. Is there a nicer way?
Python Solutions
Solution 1 - Python
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]
iter(())
?
Why not just use 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.
Solution 2 - Python
iter(())
You don't require a generator. C'mon guys!
Solution 3 - Python
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.
Solution 4 - Python
Another option is:
(_ for _ in ())
Solution 5 - Python
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.
Solution 6 - Python
Must it be a generator function? If not, how about
def f():
return iter(())
Solution 7 - Python
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
Solution 8 - Python
generator = (item for item in [])
Solution 9 - Python
I want to give a class based example since we haven't had any suggested yet. This is a callable iterator that generates no items. I believe this is a straightforward and descriptive way to solve the issue.
class EmptyGenerator:
def __iter__(self):
return self
def __next__(self):
raise StopIteration
>>> list(EmptyGenerator())
[]
Solution 10 - Python
Nobody has mentioned it yet, but calling the built-in function zip
with no arguments returns an empty iterator:
>>> it = zip()
>>> next(it)
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
StopIteration