On 11Jun2021 19:02, Jach Fong <jf...@ms4.hinet.net> wrote:
>>>> def foo():
>... # do something
>...
>>>> a = []
>>>> for i in range(3):
>... a.append(foo())
>...
>>>> a
>[]
>>>>
Chris has explained that what you've written will never do it. But if
instead you wanted to filter out None results, getting an empty list
from the filter, then the filter() builtin function will do you.
First, as Chris alluded to, all functions return values. Those with a
"bare" return (no expression) or which just fall off the end of the
function body actaully return None.
There's the obvious:
a = []
for i in range(3):
item = foo()
if item is not None:
a.append(item)
Using a filter... "help(filter)" says:
>>> help(filter)
Help on class filter in module builtins:
class filter(object)
| filter(function or None, iterable) --> filter object
|
| Return an iterator yielding those items of iterable for which
function(item)
| is true. If function is None, return the items that are true.
|
| Methods defined here:
|
| __getattribute__(self, name, /)
| Return getattr(self, name).
|
| __iter__(self, /)
| Implement iter(self).
|
| __next__(self, /)
| Implement next(self).
|
| __reduce__(...)
| Return state information for pickling.
|
| ----------------------------------------------------------------------
| Static methods defined here:
|
| __new__(*args, **kwargs) from builtins.type
| Create and return a new object. See help(type) for accurate
signature.
which is more that I expected. Usage:
a = list(filter(lambda item: item is not None, items))
where items is the various returns from foo().
Or you could just write a list comprehension directly:
a = [ item for item in items if items is not None ]
You want items to be your calls to foo(). Eg:
a = [ item for item in (foo() for _ in range(3)) if items is not None ]
The (expression for name in iterable) is a generator expression. Like a
list comprehension but it is a generator - it only runs as it is
consumed. A list comprehension makes an actual list (stores all the
results in the list).
Cheers,
Cameron Simpson <c...@cskk.id.au>
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