Python idiom to return first item or None
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I'm sure there's a simpler way of doing this that's just not occurring to me.
I'm calling a bunch of methods that return a list. The list may be empty. If the list is non-empty, I want to return the first item; otherwise, I want to return None. This code works:
my_list = get_list() if len(my_list) > 0: return my_list return None
It seems to me that there should be a simple one-line idiom for doing this, but for the life of me I can't think of it. Is there?
The reason that I'm looking for a one-line expression here is not that I like incredibly terse code, but because I'm having to write a lot of code like this:
x = get_first_list() if x: # do something with x # inevitably forget the  part, and have a bug to fix y = get_second_list() if y: # do something with y # inevitably forget the  part AGAIN, and have another bug to fix
What I'd like to be doing can certainly be accomplished with a function (and probably will be):
def first_item(list_or_none): if list_or_none: return list_or_none x = first_item(get_first_list()) if x: # do something with x y = first_item(get_second_list()) if y: # do something with y
I posted the question because I'm frequently surprised by what simple expressions in Python can do, and I thought that writing a function was a silly thing to do if there was a simple expression could do the trick. But seeing these answers, it seems like a function is the simple solution.
your_list can be
next(iter(your_list or ), None)
def get_first(iterable, default=None): if iterable: for item in iterable: return item return default
x = get_first(get_first_list()) if x: ... y = get_first(get_second_list()) if y: ...
Another option is to inline the above function:
for x in get_first_list() or : # process x break # process at most one item for y in get_second_list() or : # process y break
break you could write:
for x in yield_first(get_first_list()): x # process x for y in yield_first(get_second_list()): y # process y
def yield_first(iterable): for item in iterable or : yield item return
Python idiom to return first item or None, The best way is to use the below-mentioned code:- a = get_list(). return a if a else None. You could also do it in one line, but it's much harder� The list may be empty. If the list is non-empty, I want to return the first item; otherwise, I want to return None. This code works: my_list = get_list() if len(my_list) > 0: return my_list return None. It seems to me that there should be a simple one-line idiom for doing this, but for the life of me, I can't think of it. Is there?
The best way is this:
a = get_list() return a if a else None
You could also do it in one line, but it's much harder for the programmer to read:
return (get_list()[:1] or [None])
Python idiom to return first item or None - Article, I'm sure there's a simpler way of doing this that's just not occurring to me. I'm calling a bunch of methods that return a list. The list may be empty. If the list is� Python idiom to return first item or None? The most Pythonic approach is what the most upvoted answer demonstrated, and it was the first thing to come to my mind when I read the question. Here's how to use it, first if the possibly empty list is passed into a function: def get_first(l): return l if l else None
(get_list() or [None])
That should work.
BTW I didn't use the variable
list, because that overwrites the builtin
Edit: I had a slightly simpler, but wrong version here earlier.
hynek/first: The function you always missed in Python , The function you always missed in Python: return the first true value of an iterable . truth value of the element or a default value if None doesn't fit your use case. However the existing solutions aren't very idiomatic for such a common and� None in list_1 Like any (), the membership test on a list will scan all elements but short-circuit by returning as soon as a match is found. any () returns True or False, never None, so your any (list_1) is None test is certainly not going anywhere. You'd have to pass in a generator expression for any () to iterate over, instead:
The most python idiomatic way is to use the next() on a iterator since list is iterable. just like what @J.F.Sebastian put in the comment on Dec 13, 2011.
next(iter(the_list), None) This returns None if
the_list is empty. see next() Python 2.6+
or if you know for sure
the_list is not empty:
iter(the_list).next() see iterator.next() Python 2.2+
Idiomatic Python — Intermediate and Advanced Software Carpentry , And that's the sort of benefit you will get from writing idiomatic Python. run cmp on each of the tuples, which sort on the first element of each tuple: into the object type, because it reminds me that it is not a generic Python list or dictionary. Often in Python, functions which return None are used like void functions in C -- Their purpose is generally to operate on the input arguments in place (unless you're using global data (shudders)). Returning None usually makes it more explicit that the arguments were mutated.
If you find yourself trying to pluck the first thing (or None) from a list comprehension you can switch to a generator to do it like:
next((x for x in blah if cond), None)
Pro: works if blah isn't indexable Con: it's unfamiliar syntax. It's useful while hacking around and filtering stuff in ipython though.
Built-in Functions — Python 3.8.5 documentation, def all(iterable): for element in iterable: if not element: return False return True New in version 3.2: This function was first removed in Python 3.0 and then brought back in Python 3.2. chr (i)� To declare a class method, use this idiom:. Return 1 if the object provides sequence protocol, and 0 otherwise. Note that it returns 1 for Python classes with a __getitem__() method unless they are dict subclasses since in general case it is impossible to determine what the type of keys it supports.
2. Built-in Functions — Python 3.4.10 documentation, def all(iterable): for element in iterable: if not element: return False return True A class method receives the class as implicit first argument, just like an instance method receives the instance. To declare a class method, use this idiom: class C: � Python | Get first and last elements of a list Sometimes, there might be a need to get the range between which a number lies in the list, for such applications we require to get the first and last element of the list.
Programming FAQ — Python 3.8.5 documentation, Why do lambdas defined in a loop with different values all return the same result? My class defines __del__ but it is not called when I delete the object. By the time the import is called, the first module will have finished initializing, and the By definition, immutable objects such as numbers, strings, tuples, and None , are � Python uses the keyword None to define null objects and variables. While None does serve some of the same purposes as null in other languages, it’s another beast entirely. As the null in Python, None is not defined to be 0 or any other value. In Python, None is an object and a first-class citizen! In this tutorial, you’ll learn:
itertools — Functions creating iterators for efficient looping — Python , r-length tuples, all possible orderings, no repeated elements Make an iterator that returns elements from the first iterable until it is exhausted, then proceeds to� Expression statements are used (mostly interactively) to compute and write a value, or (usually) to call a procedure (a function that returns no meaningful result; in Python, procedures return the value None). Other uses of expression statements are allowed and occasionally useful. The syntax for an expression statement is:
- btw, on Python 2.6+ you could use
next(iter(your_list), None)instead of
get_second_list()must always return an iterable).
- I think you mean
next(iter(your_list)), since if you supply a second argument to
iter, you're telling it that the first argument is callable.
Nonehere is the second parameter for
next()returns its second parameter if given instead of raising
- Solution suggested by @J.F.Sebastian exists with duplicate question: stackoverflow.com/a/18533669/144408
- That last option is almost exactly what I'm looking for: it's clear, it works, and it doesn't require me to define a new function. I'd say "exactly" if the break were somehow not needed, because the risk of omitting it is not insignificant. But this approach has the ring of truth.
- Oh, I don't like this at all. If any item in the list evaluates False, that value is discarded and replaced. If you have an empty string
""in the list, that is discarded and replaced by an empty list
. If you have a 0, also replaced by
. If you have
Falsein there, also replaced. Etc. You might get away with this in a specific case, but this is a bad habit to develop.
bool(lst)tells us whether
len(lst) > 0it doesn't tell us anything about what items
bool([False]) == True;therefore the expression
[False] or returns
[False], the same is for
 or it returns
- @RobertRossney: I've added
- @ThomasAhle: both are true. I've updated the answer to provide the solution for non-None case.
- Oops. Should have mentioned this is a Python 2.4 app.
- @Adam: the future may use
- @J.F.Sebastian actually
- @BradleagheJohnson: wrong. It raises StopIteration instead of returning
Noneas OP asks. Try:
- This is clever and works, but I think "return foo if foo else None" as suggested by efotinis is a little easier to follow for maintenance.