How do I check if a list is empty

Question

For example  if passed the following   a        How do I check to see if a is empty

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Simply use is empty   or make function like -   def is empty any structure       if any structure          print  Structure is not empty            return True     else          print  Structure is empty            return False     It can be used for any data structure like a list tuples  dictionary and many more  By these  you can call it many times using just is empty any structure

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Patrick s  accepted  answer is right  if not a  is the right way to do it  Harley Holcombe s answer is right that this is in the PEP 8 style guide  But what none of the answers explain is why it s a good idea to follow the idiom   even if you personally find it s not explicit enough or confusing to Ruby users or whatever   Python code  and the Python community  has very strong idioms  Following those idioms makes your code easier to read for anyone experienced in Python  And when you violate those idioms  that s a strong signal   It s true that if not a  doesn t distinguish empty lists from None  or numeric 0  or empty tuples  or empty user-created collection types  or empty user-created not-quite-collection types  or single-element NumPy array acting as scalars with falsey values  etc  And sometimes it s important to be explicit about that  And in that case  you know what you want to be explicit about  so you can test for exactly that  For example  if not a and a is not None  means  anything falsey except None   while if len a     0  means  only empty sequences   and anything besides a sequence is an error here   and so on  Besides testing for exactly what you want to test  this also signals to the reader that this test is important   But when you don t have anything to be explicit about  anything other than if not a  is misleading the reader  You re signaling something as important when it isn t   You may also be making the code less flexible  or slower  or whatever  but that s all less important   And if you habitually mislead the reader like this  then when you do need to make a distinction  it s going to pass unnoticed because you ve been  crying wolf  all over your code

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we could use a simple if else   item list    if len item list     0      print  list is empty   else      print  list is not empty

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Method 1  Preferred    if not a       print   Empty      Method 2    if len a     0      print   Empty      Method 3    if a           print   Empty

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This is the first google hit for  python test empty array  and similar queries  plus other people seem to be generalizing the question beyond just lists  so I thought I d add a caveat for a different type of sequence that a lot of people might use   Other methods don t work for NumPy arrays  You need to be careful with NumPy arrays  because other methods that work fine for lists or other standard containers fail for NumPy arrays   I explain why below  but in short  the preferred method is to use size   The  pythonic  way doesn t work  Part 1  The  pythonic  way fails with NumPy arrays because NumPy tries to cast the array to an array of bools  and if x tries to evaluate all of those bools at once for some kind of aggregate truth value   But this doesn t make any sense  so you get a ValueError    gt  gt  gt  x   numpy array  0 1    gt  gt  gt  if x  print  x   ValueError  The truth value of an array with more than one element is ambiguous  Use a any   or a all     The  pythonic  way doesn t work  Part 2  But at least the case above tells you that it failed   If you happen to have a NumPy array with exactly one element  the if statement will  work   in the sense that you don t get an error   However  if that one element happens to be 0  or 0 0  or False        the if statement will incorrectly result in False    gt  gt  gt  x   numpy array  0     gt  gt  gt  if x  print  x       else  print  No x   No x   But clearly x exists and is not empty   This result is not what you wanted   Using len can give unexpected results  For example   len  numpy zeros  1 0       returns 1  even though the array has zero elements   The numpythonic way  As explained in the SciPy FAQ  the correct method in all cases where you know you have a NumPy array is to use if x size    gt  gt  gt  x   numpy array  0 1    gt  gt  gt  if x size  print  x   x   gt  gt  gt  x   numpy array  0     gt  gt  gt  if x size  print  x       else  print  No x   x   gt  gt  gt  x   numpy zeros  1 0    gt  gt  gt  if x size  print  x       else  print  No x   No x   If you re not sure whether it might be a list  a NumPy array  or something else  you could combine this approach with the answer  dubiousjim gives to make sure the right test is used for each type   Not very  pythonic   but it turns out that NumPy intentionally broke pythonicity in at least this sense   If you need to do more than just check if the input is empty  and you re using other NumPy features like indexing or math operations  it s probably more efficient  and certainly more common  to force the input to be a NumPy array   There are a few nice functions for doing this quickly      most importantly numpy asarray   This takes your input  does nothing if it s already an array  or wraps your input into an array if it s a list  tuple  etc   and optionally converts it to your chosen dtype   So it s very quick whenever it can be  and it ensures that you just get to assume the input is a NumPy array   We usually even just use the same name  as the conversion to an array won t make it back outside of the current scope   x   numpy asarray x  dtype numpy double    This will make the x size check work in all cases I see on this page

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What brought me here is a special use-case  I actually wanted a function to tell me if a list is empty or not  I wanted to avoid writing my own function or using a lambda-expression here  because it seemed like it should be simple enough    foo   itertools takewhile is not empty   f x  for x in itertools count 1      And  of course  there is a very natural way to do it   foo   itertools takewhile bool   f x  for x in itertools count 1      Of course  do not use bool in if  i e   if bool L    because it s implied  But  for the cases when  is not empty  is explicitly needed as a function  bool is the best choice

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if not a    print  List is empty     Using the implicit booleanness of the empty list is quite pythonic

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I prefer the following   if a           print  The list is empty

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Here are a few ways you can check if a list is empty   a       the list   1  The pretty simple pythonic way   if not a      print  a is empty     In Python  empty containers such as lists tuples sets dicts variables etc are seen as False  One could simply treat the list as a predicate  returning a Boolean value   And  a True value would indicate that it s non-empty   2  A much explicit way  using the len   to find the length and check if it equals to 0   if len a     0      print  a is empty     3  Or comparing it to an anonymous empty list   if a            print  a is empty     4  Another yet silly way to do is using exception and iter     try      next iter a         list has elements except StopIteration      print  Error  a is empty

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print  not empty  if a else  empty     a little more practical   a pop   if a else None   and shertest version   if a  a pop

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You can even try using bool   like this  Although it is less readable surely it s a concise way to perform this      a    1 2 3       print bool a     it will return True     a           print bool a     it will return False  I love this way for the checking list is empty or not  Very handy and useful

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def list test  L       if   L is None    print  list is None       elif not L        print  list is empty       else  print  list has  d elements    len L    list test None  list test     list test  1 2 3     It is sometimes good to test for None and for emptiness separately as those are two different states  The code above produces the following output   list is None  list is empty  list has 3 elements   Although it s worth nothing that None is falsy  So if you don t want to separate test for None-ness  you don t have to do that    def list test2  L       if not L        print  list is empty       else  print  list has  d elements    len L    list test2 None  list test2     list test2  1 2 3     produces expected  list is empty list is empty list has 3 elements

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From python3 onwards you can use  a         to check if the list is empty  EDIT   This works with python2 7 too     I am not sure why there are so many complicated answers  It s pretty clear and straightforward

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Many answers have been given  and a lot of them are pretty good  I just wanted to add that the check  not a   will also pass for None and other types of empty structures  If you truly want to check for an empty list  you can do this   if isinstance a  list  and len a   0      print  Received an empty list

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I had written   if isinstance a   list  some  other  types  i  accept   and not a      do stuff   which was voted -1  I m not sure if that s because readers objected to the strategy or thought the answer wasn t helpful as presented  I ll pretend it was the latter  since---whatever counts as  pythonic ---this is the correct strategy  Unless you ve already ruled out  or are prepared to handle cases where a is  for example  False  you need a test more restrictive than just if not a   You could use something like this   if isinstance a  numpy ndarray  and not a size      do stuff elif isinstance a  collections Sized  and not a      do stuff   the first test is in response to  Mike s answer  above  The third line could also be replaced with   elif isinstance a   list  tuple   and not a    if you only want to accept instances of particular types  and their subtypes   or with   elif isinstance a   list  tuple   and not len a     You can get away without the explicit type check  but only if the surrounding context already assures you that a is a value of the types you re prepared to handle  or if you re sure that types you re not prepared to handle are going to raise errors  e g   a TypeError if you call len on a value for which it s undefined  that you re prepared to handle  In general  the  pythonic  conventions seem to go this last way  Squeeze it like a duck and let it raise a DuckError if it doesn t know how to quack  You still have to think about what type assumptions you re making  though  and whether the cases you re not prepared to handle properly really are going to error out in the right places  The Numpy arrays are a good example where just blindly relying on len or the boolean typecast may not do precisely what you re expecting

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The truth value of an empty list is False whereas for a non-empty list it is True

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If you want to check if a list is empty   l      if l        do your stuff    If you want to check whether all the values in list is empty  However it will be True for an empty list   l        False  0                  if all bool x  for x in l         do your stuff    If you want to use both cases together   def empty list lst       if len lst     0          return False     else          return all bool x  for x in l    Now you can use   if empty list lst         do your stuff

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Best way to check if a list is empty      For example  if passed the following   a            How do I check to see if a is empty    Short Answer   Place the list in a boolean context  for example  with an if or while statement   It will test False if it is empty  and True otherwise  For example   if not a                              do this      print  a is an empty list     PEP 8  PEP 8  the official Python style guide for Python code in Python s standard library  asserts      For sequences   strings  lists  tuples   use the fact that empty sequences are false   Yes  if not seq       if seq   No  if len seq       if not len seq      We should expect that standard library code should be as performant and correct as possible  But why is that the case  and why do we need this guidance   Explanation  I frequently see code like this from experienced programmers new to Python   if len a     0                        Don t do this      print  a is an empty list     And users of lazy languages may be tempted to do this   if a                                  Don t do this      print  a is an empty list     These are correct in their respective other languages  And this is even semantically correct in Python    But we consider it un-Pythonic because Python supports these semantics directly in the list object s interface via boolean coercion   From the docs  and note specifically the inclusion of the empty list           By default  an object is considered true unless its class defines   either a   bool     method that returns False or a   len     method   that returns zero  when called with the object  Here are most of the built-in objects considered false          constants defined to be false  None and False    zero of any numeric type  0  0 0  0j  Decimal 0   Fraction 0  1    empty sequences and collections                  set    range 0       And the datamodel documentation      object   bool   self       Called to implement truth value testing and the built-in operation bool    should return False or True  When this method is not defined      len     is called  if it is defined  and the object is considered true if its result is nonzero  If a class defines neither   len       nor   bool      all its instances are considered true    and      object   len   self       Called to implement the built-in function len    Should return the length of the object  an integer    0  Also  an object that doesn   t define a   bool     method and whose   len     method returns zero is considered to be false in a Boolean context    So instead of this   if len a     0                        Don t do this      print  a is an empty list     or this   if a                              Don t do this      print  a is an empty list     Do this   if not a      print  a is an empty list     Doing what s Pythonic usually pays off in performance   Does it pay off   Note that less time to perform an equivalent operation is better     gt  gt  gt  import timeit  gt  gt  gt  min timeit repeat lambda  len        0  repeat 100   0 13775854044661884  gt  gt  gt  min timeit repeat lambda            repeat 100   0 0984637276455409  gt  gt  gt  min timeit repeat lambda  not     repeat 100   0 07878462291455435   For scale  here s the cost of calling the function and constructing and returning an empty list  which you might subtract from the costs of the emptiness checks used above    gt  gt  gt  min timeit repeat lambda      repeat 100   0 07074015751817342   We see that either checking for length with the builtin function len compared to 0 or checking against an empty list is much less performant than using the builtin syntax of the language as documented   Why   For the len a     0 check   First Python has to check the globals to see if len is shadowed    Then it must call the function  load 0  and do the equality comparison in Python  instead of with C     gt  gt  gt  import dis  gt  gt  gt  dis dis lambda  len        0    1           0 LOAD GLOBAL              0  len                2 BUILD LIST               0               4 CALL FUNCTION            1               6 LOAD CONST               1  0                8 COMPARE OP               2                   10 RETURN VALUE   And for the          it has to build an unnecessary list and then  again  do the comparison operation in Python s virtual machine  as opposed to C    gt  gt  gt  dis dis lambda              1           0 BUILD LIST               0               2 BUILD LIST               0               4 COMPARE OP               2                    6 RETURN VALUE   The  Pythonic  way is a much simpler and faster check since the length of the list is cached in the object instance header    gt  gt  gt  dis dis lambda  not       1           0 BUILD LIST               0               2 UNARY NOT               4 RETURN VALUE   Evidence from the C source and documentation     PyVarObject      This is an extension of PyObject that adds the ob size field  This is only used for objects that have some notion of length  This type does not often appear in the Python C API  It corresponds to the fields defined by the expansion of the PyObject VAR HEAD macro    From the c source in Include listobject h   typedef struct       PyObject VAR HEAD        Vector of pointers to list elements   list 0  is ob item 0   etc         PyObject   ob item          ob item contains space for  allocated  elements   The number        currently in use is ob size         Invariants             0  lt   ob size  lt   allocated            len list     ob size   Response to comments      I would point out that this is also true for the non-empty case though its pretty ugly as with l    then  timeit len l     0 90 6 ns    8 3 ns   timeit l       55 6 ns    3 09   timeit not not l 38 5 ns    0 372  But there is no way anyone is going to enjoy not not l despite triple the speed  It looks ridiculous  But the speed wins out   I suppose the problem is testing with timeit since just if l  is sufficient but surprisingly  timeit bool l  yields 101 ns    2 64 ns  Interesting there is no way to coerce to bool without this penalty   timeit l is useless since no conversion would occur    IPython magic   timeit  is not entirely useless here   In  1   l                                                                         In  2    timeit l                                                                20 ns    0 155 ns per loop  mean    std  dev  of 7 runs  100000000 loops each   In  3    timeit not l                                                            24 4 ns    1 58 ns per loop  mean    std  dev  of 7 runs  10000000 loops each   In  4    timeit not not l                                                        30 1 ns    2 16 ns per loop  mean    std  dev  of 7 runs  10000000 loops each    We can see there s a bit of linear cost for each additional not here  We want to see the costs  ceteris paribus  that is  all else equal - where all else is minimized as far as possible   In  5    timeit if l  pass                                                       22 6 ns    0 963 ns per loop  mean    std  dev  of 7 runs  10000000 loops each   In  6    timeit if not l  pass                                                   24 4 ns    0 796 ns per loop  mean    std  dev  of 7 runs  10000000 loops each   In  7    timeit if not not l  pass                                               23 4 ns    0 793 ns per loop  mean    std  dev  of 7 runs  10000000 loops each    Now let s look at the case for an unempty list   In  8   l    1                                                                    In  9    timeit if l  pass                                                       23 7 ns    1 06 ns per loop  mean    std  dev  of 7 runs  10000000 loops each   In  10    timeit if not l  pass                                                  23 6 ns    1 64 ns per loop  mean    std  dev  of 7 runs  10000000 loops each   In  11    timeit if not not l  pass                                              26 3 ns    1 ns per loop  mean    std  dev  of 7 runs  10000000 loops each    What we can see here is that it makes little difference whether you pass in an actual bool to the condition check or the list itself  and if anything  giving the list  as is  is faster   Python is written in C  it uses its logic at the C level  Anything you write in Python will be slower  And it will likely be orders of magnitude slower unless you re using the mechanisms built into Python directly

User · Answer

Why check at all  No one seems to have addressed questioning your need to test the list in the first place   Because you provided no additional context  I can imagine that you may not need to do this check in the first place  but are unfamiliar with list processing in Python  I would argue that the most pythonic way is to not check at all  but rather to just process the list   That way it will do the right thing whether empty or full  a       for item in a       lt do something with item gt    lt rest of code gt   This has the benefit of handling any contents of a  while not requiring a specific check for emptiness   If a is empty  the dependent block will not execute and the interpreter will fall through to the next line  If you do actually need to check the array for emptiness  the other answers are sufficient

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To check whether a list is empty or not you can use two following ways  But remember  we should avoid the way of explicitly checking for a type of sequence  it s a less pythonic way     def enquiry list1        if len list1     0           return 0     else           return 1                                                                                                      list1        if enquiry list1        print   The list isn t empty    else       print  The list is Empty       Result   The list is Empty        The second way is a more pythonic one  This method is an implicit way of checking and much more preferable than the previous one    def enquiry list1        if not list1           return True     else           return False                                                                                                      list1        if enquiry list1        print   The list is Empty    else       print   The list isn t empty       Result   The list is Empty    Hope this helps

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Simple way is checking the length is equal zero   if len a     0      print  a is empty

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I prefer it explicitly   if len li     0      print  the list is empty     This way it s 100  clear that li is a sequence  list  and we want to test its size  My problem with if not li      is that it gives the false impression that li is a boolean variable

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len   is an O 1  operation for Python lists  strings  dicts  and sets  Python internally keeps track of the number of elements in these containers   JavaScript has a similar notion of truthy falsy

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Being inspired by  dubiousjim s solution  I propose to use an additional general check of whether is it something iterable  import collections def is empty a       return not a and isinstance a  collections Iterable    Note  a string is considered to be iterable  - add and not isinstance a  str unicode   if you want the empty string to be excluded  Test    gt  gt  gt  is empty  sss   False  gt  gt  gt  is empty 555  False  gt  gt  gt  is empty 0  False  gt  gt  gt  is empty     True  gt  gt  gt  is empty  3   False  gt  gt  gt  is empty     True  gt  gt  gt  is empty     True  gt  gt  gt  is empty     True

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From documentation on truth value testing   All values other than what is listed here are considered True   None False zero of any numeric type  for example  0  0 0  0j  any empty sequence  for example              any empty mapping  for example      instances of user-defined classes  if the class defines a   bool     or   len     method  when that method returns the integer zero or bool value False    As can be seen  empty list    is falsy  so doing what would be done to a boolean value sounds most efficient   if not a      print   a  is empty

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The pythonic way to do it is from the PEP 8 style guide  where Yes means    recommended    and No means    not recommended          For sequences   strings  lists  tuples   use the fact that empty sequences are false      Yes  if not seq       if seq   No   if len seq        if not len seq

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An empty list is itself considered false in true value testing  see python documentation    a      if a       print  not empty     Daren Thomas     EDIT  Another point against testing   the empty list as False  What about   polymorphism  You shouldn t depend on   a list being a list  It should just   quack like a duck - how are you going   to get your duckCollection to quack     False   when it has no elements    Your duckCollection should implement   nonzero   or   len   so the if a  will work without problems

[python] How do I check if a list is empty?

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