Test if a variable is a list or tuple

Question

In python  what s the best way to test if a variable contains a list or a tuple   ie  a collection   Is isinstance   as evil as suggested here  http   www canonical org  kragen isinstance   Update  the most common reason I want to distinguish a list from a string is when I have some indefinitely deep nested tree   data-structure of lists of lists of lists of strings etc  which I m exploring with a recursive algorithm and I need to know when I ve hit the  leaf  nodes

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If you just need to know if you can use the foo 123  notation with the variable  you can check for the existence of a   getitem   attribute  which is what python calls when you access by index  with hasattr foo     getitem

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Not the most elegant  but I do  for Python 3   if hasattr instance     iter     and not isinstance instance   str  bytes             This allows other iterables  like Django querysets  but excludes strings and bytestrings   I typically use this in functions that accept either a single object ID or a list of object IDs   Sometimes the object IDs can be strings and I don t want to iterate over those character by character

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On Python 2 8 type list  is list returns false   I would suggest comparing the type in this horrible way   if type a     type         print  variable a is a list     well at least on my system  using anaconda on Mac OS X Yosemite

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Go ahead and use isinstance if you need it   It is somewhat evil  as it excludes custom sequences  iterators  and other things that you might actually need   However  sometimes you need to behave differently if someone  for instance  passes a string   My preference there would be to explicitly check for str or unicode like so   import types isinstance var  types StringTypes    N B  Don t mistake types StringType for types StringTypes   The latter incorporates str and unicode objects   The types module is considered by many to be obsolete in favor of just checking directly against the object s type  so if you d rather not use the above  you can alternatively check explicitly against str and unicode  like this   isinstance var   str  unicode      Edit   Better still is   isinstance var  basestring    End edit  After either of these  you can fall back to behaving as if you re getting a normal sequence  letting non-sequences raise appropriate exceptions   See the thing that s  evil  about type checking is not that you might want to behave differently for a certain type of object  it s that you artificially restrict your function from doing the right thing with unexpected object types that would otherwise do the right thing   If you have a final fallback that is not type-checked  you remove this restriction   It should be noted that too much type checking is a code smell that indicates that you might want to do some refactoring  but that doesn t necessarily mean you should avoid it from the getgo

User · Answer

How about  hasattr a     iter        It tells if the object returned can be iterated over as a generator   By default  tuples and lists can  but not the string types

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Document the argument as needing to be a sequence  and use it as a sequence  Don t check the type

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Has to be more complex test if you really want to handle just about anything as function argument   type a     type     and hasattr a     iter       Although  usually it s enough to just spell out that a function expects iterable  and then check only type a     type        Also it may happen that for a string you have a simple processing path or you are going to be nice and do a split etc   so you don t want to yell at strings and if someone sends you something weird  just let him have an exception

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gt  gt  gt  l       gt  gt  gt  l   class     name   in   list    tuple   True

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There s nothing wrong with using isinstance as long as it s not redundant  If a variable should only be a list tuple then document the interface and just use it as such  Otherwise a check is perfectly reasonable   if isinstance a  collections Iterable         use as a container else        not a container    This type of check does have some good use-cases  such as with the standard string startswith   endswith methods  although to be accurate these are implemented in C in CPython using an explicit check to see if it s a tuple - there s more than one way to solve this problem  as mentioned in the article you link to    An explicit check is often better than trying to use the object as a container and handling the exception - that can cause all sorts of problems with code being run partially or unnecessarily

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Another easy way to find out if a variable is either list or tuple or generally check variable type would be        def islist obj            if   list  in str type obj      return True          else   return False

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In principle  I agree with Ignacio  above  but you can also use type to check if something is a tuple or a list    gt  gt  gt  a    1    gt  gt  gt  type a   type  tuple    gt  gt  gt  a    1   gt  gt  gt  type a   type  list

User · Answer

if type x  is list      print  a list  elif type x  is tuple      print  a tuple  else      print  neither a tuple or a list

User · Answer

Python uses  Duck typing   i e  if a variable kwaks like a duck  it must be a duck  In your case  you probably want it to be iterable  or you want to access the item at a certain index  You should just do this  i e  use the object in for var  or var idx  inside a try block  and if you get an exception it wasn t a duck

[python] Test if a variable is a list or tuple

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