How to check if an object is a list or tuple but not string

Question

This is what I normally do in order to ascertain that the input is a list tuple - but not a str  Because many times I stumbled upon bugs where a function passes a str object by mistake  and the target function does for x in lst assuming that lst is actually a list or tuple   assert isinstance lst   list  tuple     My question is  is there a better way of achieving this

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I do this in my testcases   def assertIsIterable self  item        add types here you don t want to mistake as iterables     if isinstance item  basestring            raise AssertionError  type  s is not iterable    type item         Fake an iteration      try          for x in item              break      except TypeError          raise AssertionError  type  s is not iterable    type item     Untested on generators  I think you are left at the next  yield  if passed in a generator  which may screw things up downstream   But then again  this is a  unittest

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Python 3 has this   from typing import List  def isit value       return isinstance value  List   isit  1  2  3      True isit  test      False isit   Hello    Mars       False isit  1  2      False   So to check for both Lists and Tuples  it would be   from typing import List  Tuple  def isit value       return isinstance value  List  or isinstance value  Tuple

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This is not intended to directly answer the OP  but I wanted to share some related ideas   I was very interested in  steveha answer above  which seemed to give an example where duck typing seems to break  On second thought  however  his example suggests that duck typing is hard to conform to  but it does not suggest that str deserves any special handling   After all  a non-str type  e g   a user-defined type that maintains some complicated recursive structures  may cause  steveha srepr function to cause an infinite recursion  While this is admittedly rather unlikely  we can t ignore this possibility  Therefore  rather than special-casing str in srepr  we should clarify what we want srepr to do when an infinite recursion results   It may seem that one reasonable approach is to simply break the recursion in srepr the moment list arg      arg   This would  in fact  completely solve the problem with str  without any isinstance   However  a really complicated recursive structure may cause an infinite loop where list arg      arg  never happens  Therefore  while the above check is useful  it s not sufficient  We need something like a hard limit on the recursion depth   My point is that if you plan to handle arbitrary argument types  handling str via duck typing is far  far easier than handling the more general types you may  theoretically  encounter  So if you feel the need to exclude str instances  you should instead demand that the argument is an instance of one of the few types that you explicitly specify

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Python with PHP flavor   def is array var       return isinstance var   list  tuple

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Remember that in Python we want to use  duck typing    So  anything that acts like a list can be treated as a list   So  don t check for the type of a list  just see if it acts like a list   But strings act like a list too  and often that is not what we want   There are times when it is even a problem   So  check explicitly for a string  but then use duck typing   Here is a function I wrote for fun   It is a special version of repr   that prints any sequence in angle brackets    lt           def srepr arg       if isinstance arg  basestring     Python 3  isinstance arg  str          return repr arg      try          return   lt          join srepr x  for x in arg      gt       except TypeError    catch when for loop fails         return repr arg    not a sequence so just return repr   This is clean and elegant  overall   But what s that isinstance   check doing there   That s kind of a hack   But it is essential   This function calls itself recursively on anything that acts like a list   If we didn t handle the string specially  then it would be treated like a list  and split up one character at a time   But then the recursive call would try to treat each character as a list -- and it would work   Even a one-character string works as a list   The function would keep on calling itself recursively until stack overflow   Functions like this one  that depend on each recursive call breaking down the work to be done  have to special-case strings--because you can t break down a string below the level of a one-character string  and even a one-character string acts like a list   Note  the try except is the cleanest way to express our intentions   But if this code were somehow time-critical  we might want to replace it with some sort of test to see if arg is a sequence   Rather than testing the type  we should probably test behaviors   If it has a  strip   method  it s a string  so don t consider it a sequence  otherwise  if it is indexable or iterable  it s a sequence   def is sequence arg       return  not hasattr arg   strip   and             hasattr arg     getitem     or             hasattr arg     iter       def srepr arg       if is sequence arg           return   lt          join srepr x  for x in arg      gt       return repr arg    EDIT  I originally wrote the above with a check for   getslice     but I noticed that in the collections module documentation  the interesting method is   getitem      this makes sense  that s how you index an object   That seems more fundamental than   getslice     so I changed the above

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assert  type lst     list     type lst     tuple    Not a valid lst type  cannot be string

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Try this for readability and best practices   Python2  import types if isinstance lst  types ListType  or isinstance lst  types TupleType         Do something   Python3  import typing if isinstance lst  typing List  or isinstance lst  typing Tuple         Do something   Hope it helps

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Another version of duck-typing to help distinguish string-like objects from other sequence-like objects   The string representation of string-like objects is the string itself  so you can check if you get an equal object back from the str constructor     If a string was passed  convert it to a single-element sequence if var    str var       my list    var     All other iterables else       my list   list var    This should work for all objects compatible with str and for all kinds of iterable objects

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Generally speaking  the fact that a function which iterates over an object works on strings as well as tuples and lists is more feature than bug   You certainly can use isinstance or duck typing to check an argument  but why should you   That sounds like a rhetorical question  but it isn t   The answer to  why should I check the argument s type   is probably going to suggest a solution to the real problem  not the perceived problem   Why is it a bug when a string is passed to the function   Also   if it s a bug when a string is passed to this function  is it also a bug if some other non-list tuple iterable is passed to it   Why  or why not   I think that the most common answer to the question is likely to be that developers who write f  abc   are expecting the function to behave as though they d written f   abc      There are probably circumstances where it makes more sense to protect developers from themselves than it does to support the use case of iterating across the characters in a string   But I d think long and hard about it first

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In python 2 only  not python 3    assert not isinstance lst  basestring    Is actually what you want  otherwise you ll miss out on a lot of things which act like lists  but aren t subclasses of list or tuple

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in python  3 6   import collections isinstance set   collections abc Container  True isinstance    collections abc Container  True isinstance    collections abc Container  True isinstance    collections abc Container  True isinstance str collections abc Container  False

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For Python 3   import collections abc  if isinstance obj  collections abc Sequence  and not isinstance obj  str       print  obj is a sequence  list  tuple  etc  but not a string        Changed in version 3 3  Moved Collections Abstract Base Classes to the collections abc module  For backwards compatibility  they will continue to be visible in this module as well until version 3 8 where it will stop working    For Python 2   import collections  if isinstance obj  collections Sequence  and not isinstance obj  basestring       print  obj is a sequence  list  tuple  etc  but not a string or unicode

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simplest way    using any and isinstance   gt  gt  gt  console routers    x   gt  gt  gt  any  isinstance console routers  list   isinstance console routers  tuple    False  gt  gt  gt   gt  gt  gt  console routers     x     gt  gt  gt  any  isinstance console routers  list   isinstance console routers  tuple    True  gt  gt  gt  console routers   list  x     gt  gt  gt  any  isinstance console routers  list   isinstance console routers  tuple    True

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I find such a function named is sequence in tensorflow    def is sequence seq        Returns a true if its input is a collections Sequence  except strings     Args      seq  an input sequence    Returns      True if the sequence is a not a string and is a collections Sequence          return  isinstance seq  collections Sequence  and not isinstance seq  six string types     And I have verified that it meets your needs

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In  duck typing  manner  how about  try      lst   lst      except TypeError       it s not a list   or  try      lst   lst      except TypeError       it s not a tuple   respectively  This avoids the isinstance   hasattr introspection stuff   You could also check vice versa   try      lst   lst      except TypeError       it s not  base string   All variants do not actually change the content of the variable  but imply a reassignment  I m unsure whether this might be undesirable under some circumstances   Interestingly  with the  in place  assignment    no TypeError would be raised in any case if lst is a list  not a tuple   That s why the assignment is done this way  Maybe someone can shed light on why that is

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Just do this  if type lst  in  list  tuple         Do stuff

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The str object doesn t have an   iter   attribute   gt  gt  gt  hasattr        iter     False    so you can do a check  assert hasattr x     iter       and this will also raise a nice AssertionError for any other non-iterable object too   Edit  As Tim mentions in the comments  this will only work in python 2 x  not 3 x

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I tend to do this  if I really  really had to    for i in some var     if type i     type list             do something with a list    elif type i     type tuple             do something with a tuple    elif type i     type str             here s your string

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H    Hello   if type H  is list or type H  is tuple         Do Something  else        Do Something

[python] How to check if an object is a list or tuple (but not string)?

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