What is the best idiomatic way to check the type of a Python variable

Question

I need to know if a variable in Python is a string or a dict   Is there anything wrong with the following code   if type x     type str         do something with a string x  elif type x     type dict         do somethting with a dict x  else      raise ValueError   Update  I accepted avisser s answer  though I will change my mind if someone explains why isinstance is preferred over type x  is    But thanks to nakedfanatic for reminding me that it s often cleaner to use a dict  as a case statement  than an if elif else series   Let me elaborate on my use case   If a variable is a string  I need to put it in a list   If it s a dict  I need a list of the unique values   Here s what I came up with   def value list x       cases    str  lambda t   t                dict  lambda t  list set t values          try          return cases type x   x      except KeyError          return None   If isinstance is preferred  how would you write this value list   function

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type dict    says  make a new dict  and then find out what its type is    It s quicker to say just dict  But if you want to just check type  a more idiomatic way is isinstance x  dict    Note  that isinstance also includes subclasses  thanks Dustin    class D dict       pass  d   D   print  type d  is dict   type d  is dict     - gt  False print  isinstance  d  dict    isinstance d  dict      - gt  True

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I think I will go for the duck typing approach -  if it walks like a duck  it quacks like a duck  its a duck   This way you will need not worry about if the string is a unicode or ascii    Here is what I will do   In  53   s  somestring   In  54   u u someunicodestring   In  55   d     In  56   for each in s u d      if hasattr each   keys            print list set each values         elif hasattr each   lower            print  each      else          print  error                                        somestring    u someunicodestring        The experts here are welcome to comment on this type of usage of ducktyping  I have been using it but got introduced to the exact concept behind it lately and am very excited about it  So I would like to know if thats an overkill to do

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I ve been using a different approach   from inspect import getmro if  type     in getmro obj   class            This is a list  or a subclass of    elif  type    in getmro obj   class            This one is a dict  or       I can t remember why I used this instead of isinstance  though

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type dict    says  make a new dict  and then find out what its type is    It s quicker to say just dict  But if you want to just check type  a more idiomatic way is isinstance x  dict    Note  that isinstance also includes subclasses  thanks Dustin    class D dict       pass  d   D   print  type d  is dict   type d  is dict     - gt  False print  isinstance  d  dict    isinstance d  dict      - gt  True

User · Answer

I think I will go for the duck typing approach -  if it walks like a duck  it quacks like a duck  its a duck   This way you will need not worry about if the string is a unicode or ascii    Here is what I will do   In  53   s  somestring   In  54   u u someunicodestring   In  55   d     In  56   for each in s u d      if hasattr each   keys            print list set each values         elif hasattr each   lower            print  each      else          print  error                                        somestring    u someunicodestring        The experts here are welcome to comment on this type of usage of ducktyping  I have been using it but got introduced to the exact concept behind it lately and am very excited about it  So I would like to know if thats an overkill to do

User · Answer

built-in types in Python have built in names    gt  gt  gt  s    hallo   gt  gt  gt  type s  is str True  gt  gt  gt  s       gt  gt  gt  type s  is dict True   btw note the is operator  However  type checking  if you want to call it that  is usually done by wrapping a type-specific test in a try-except clause  as it s not so much the type of the variable that s important  but whether you can do a certain something with it or not

User · Answer

I ve been using a different approach   from inspect import getmro if  type     in getmro obj   class            This is a list  or a subclass of    elif  type    in getmro obj   class            This one is a dict  or       I can t remember why I used this instead of isinstance  though

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I think it might be preferred to actually do  if isinstance x  str       do something with a string x  elif isinstance x  dict       do somethting with a dict x  else      raise ValueError   2 Alternate forms  depending on your code one or the other is probably considered better than that even   One is to not look before you leap  try    one  two   tupleOrValue except TypeError    one   tupleOrValue   two   None   The other approach is from Guido and is a form of function overloading which leaves your code more open ended   http   www artima com weblogs viewpost jsp thread 155514

User · Answer

isinstance is preferrable over type because it also evaluates as True when you compare an object instance with it s superclass  which basically means you won t ever have to special-case your old code for using it with dict or str subclasses   For example     gt  gt  gt  class a dict dict            pass         gt  gt  gt  type a dict       type dict     False   gt  gt  gt  isinstance a dict    dict   True   gt  gt  gt     Of course  there might be situations where you wouldn t want this behavior  but those are    hopefully    a lot less common than situations where you do want it

User · Answer

isinstance is preferrable over type because it also evaluates as True when you compare an object instance with it s superclass  which basically means you won t ever have to special-case your old code for using it with dict or str subclasses   For example     gt  gt  gt  class a dict dict            pass         gt  gt  gt  type a dict       type dict     False   gt  gt  gt  isinstance a dict    dict   True   gt  gt  gt     Of course  there might be situations where you wouldn t want this behavior  but those are    hopefully    a lot less common than situations where you do want it

User · Answer

I think it might be preferred to actually do  if isinstance x  str       do something with a string x  elif isinstance x  dict       do somethting with a dict x  else      raise ValueError   2 Alternate forms  depending on your code one or the other is probably considered better than that even   One is to not look before you leap  try    one  two   tupleOrValue except TypeError    one   tupleOrValue   two   None   The other approach is from Guido and is a form of function overloading which leaves your code more open ended   http   www artima com weblogs viewpost jsp thread 155514

User · Answer

built-in types in Python have built in names    gt  gt  gt  s    hallo   gt  gt  gt  type s  is str True  gt  gt  gt  s       gt  gt  gt  type s  is dict True   btw note the is operator  However  type checking  if you want to call it that  is usually done by wrapping a type-specific test in a try-except clause  as it s not so much the type of the variable that s important  but whether you can do a certain something with it or not

User · Answer

What happens if somebody passes a unicode string to your function  Or a class derived from dict  Or a class implementing a dict-like interface  Following code covers first two cases  If you are using Python 2 6 you might want to use collections Mapping instead of dict as per the ABC PEP   def value list x       if isinstance x  dict           return list set x values         elif isinstance x  basestring           return  x      else          return None

User · Answer

That should work - so no  there is nothing wrong with your code  However  it could also be done with a dict    type str     do something with a string   type dict     do something with a dict  get type x   errorhandler      A bit more concise and pythonic wouldn t you say     Edit   Heeding Avisser s advice  the code also works like this  and looks nicer    str  do something with a string   dict  do something with a dict  get type x   errorhandler

User · Answer

I ve been using a different approach   from inspect import getmro if  type     in getmro obj   class            This is a list  or a subclass of    elif  type    in getmro obj   class            This one is a dict  or       I can t remember why I used this instead of isinstance  though

User · Answer

What happens if somebody passes a unicode string to your function  Or a class derived from dict  Or a class implementing a dict-like interface  Following code covers first two cases  If you are using Python 2 6 you might want to use collections Mapping instead of dict as per the ABC PEP   def value list x       if isinstance x  dict           return list set x values         elif isinstance x  basestring           return  x      else          return None

User · Answer

sigh   No  typechecking arguments in python is not necessary  It is never  necessary   If your code accepts either a string or a dict object  your design is broken   That comes from the fact that if you don t know already the type of an object in your own program  then you re doing something wrong already   Typechecking hurts code reuse and reduces performance  Having a function that performs different things depending on the type of the object passed is  bug-prone and has a behavior harder to understand and maintain   You have the following saner options   1  Make a function unique values that converts dicts in unique lists of values   def unique values some dict       return list set some dict values       Make your function assume the argument passed is always a list  That way  if you need to pass a string to the function  you just do   myfunction  some string     If you need to pass it a dict  you do   myfunction unique values some dict     That s your best option  it is clean  easy to understand and maintain  Anyone reading the code immediatelly understands what is happening  and you don t have to typecheck   2  Make two functions  one that accepts lists of strings and one that accepts  dicts  You can make one call the other internally  in the most convenient  way  myfunction dict can create a list of strings and call myfunction list    In any case  don t typecheck  It is completely unnecessary and has only  downsides  Refactor your code instead in a way you don t need to typecheck   You only get benefits in doing so  both in short and long run

User · Answer

You may want to check out typecheck  http   pypi python org pypi typecheck  Type-checking module for Python  This package provides powerful run-time typechecking facilities for Python functions  methods and generators  Without requiring a custom preprocessor or alterations to the language  the typecheck package allows programmers and quality assurance engineers to make precise assertions about the input to  and output from  their code

User · Answer

I ve been using a different approach   from inspect import getmro if  type     in getmro obj   class            This is a list  or a subclass of    elif  type    in getmro obj   class            This one is a dict  or       I can t remember why I used this instead of isinstance  though

User · Answer

You may want to check out typecheck  http   pypi python org pypi typecheck  Type-checking module for Python  This package provides powerful run-time typechecking facilities for Python functions  methods and generators  Without requiring a custom preprocessor or alterations to the language  the typecheck package allows programmers and quality assurance engineers to make precise assertions about the input to  and output from  their code

User · Answer

That should work - so no  there is nothing wrong with your code  However  it could also be done with a dict    type str     do something with a string   type dict     do something with a dict  get type x   errorhandler      A bit more concise and pythonic wouldn t you say     Edit   Heeding Avisser s advice  the code also works like this  and looks nicer    str  do something with a string   dict  do something with a dict  get type x   errorhandler

User · Answer

I think it might be preferred to actually do  if isinstance x  str       do something with a string x  elif isinstance x  dict       do somethting with a dict x  else      raise ValueError   2 Alternate forms  depending on your code one or the other is probably considered better than that even   One is to not look before you leap  try    one  two   tupleOrValue except TypeError    one   tupleOrValue   two   None   The other approach is from Guido and is a form of function overloading which leaves your code more open ended   http   www artima com weblogs viewpost jsp thread 155514

User · Answer

type dict    says  make a new dict  and then find out what its type is    It s quicker to say just dict  But if you want to just check type  a more idiomatic way is isinstance x  dict    Note  that isinstance also includes subclasses  thanks Dustin    class D dict       pass  d   D   print  type d  is dict   type d  is dict     - gt  False print  isinstance  d  dict    isinstance d  dict      - gt  True

User · Answer

isinstance is preferrable over type because it also evaluates as True when you compare an object instance with it s superclass  which basically means you won t ever have to special-case your old code for using it with dict or str subclasses   For example     gt  gt  gt  class a dict dict            pass         gt  gt  gt  type a dict       type dict     False   gt  gt  gt  isinstance a dict    dict   True   gt  gt  gt     Of course  there might be situations where you wouldn t want this behavior  but those are    hopefully    a lot less common than situations where you do want it

User · Answer

type dict    says  make a new dict  and then find out what its type is    It s quicker to say just dict  But if you want to just check type  a more idiomatic way is isinstance x  dict    Note  that isinstance also includes subclasses  thanks Dustin    class D dict       pass  d   D   print  type d  is dict   type d  is dict     - gt  False print  isinstance  d  dict    isinstance d  dict      - gt  True

User · Answer

I think I will go for the duck typing approach -  if it walks like a duck  it quacks like a duck  its a duck   This way you will need not worry about if the string is a unicode or ascii    Here is what I will do   In  53   s  somestring   In  54   u u someunicodestring   In  55   d     In  56   for each in s u d      if hasattr each   keys            print list set each values         elif hasattr each   lower            print  each      else          print  error                                        somestring    u someunicodestring        The experts here are welcome to comment on this type of usage of ducktyping  I have been using it but got introduced to the exact concept behind it lately and am very excited about it  So I would like to know if thats an overkill to do

User · Answer

isinstance is preferrable over type because it also evaluates as True when you compare an object instance with it s superclass  which basically means you won t ever have to special-case your old code for using it with dict or str subclasses   For example     gt  gt  gt  class a dict dict            pass         gt  gt  gt  type a dict       type dict     False   gt  gt  gt  isinstance a dict    dict   True   gt  gt  gt     Of course  there might be situations where you wouldn t want this behavior  but those are    hopefully    a lot less common than situations where you do want it

User · Answer

I think I will go for the duck typing approach -  if it walks like a duck  it quacks like a duck  its a duck   This way you will need not worry about if the string is a unicode or ascii    Here is what I will do   In  53   s  somestring   In  54   u u someunicodestring   In  55   d     In  56   for each in s u d      if hasattr each   keys            print list set each values         elif hasattr each   lower            print  each      else          print  error                                        somestring    u someunicodestring        The experts here are welcome to comment on this type of usage of ducktyping  I have been using it but got introduced to the exact concept behind it lately and am very excited about it  So I would like to know if thats an overkill to do

User · Answer

What happens if somebody passes a unicode string to your function  Or a class derived from dict  Or a class implementing a dict-like interface  Following code covers first two cases  If you are using Python 2 6 you might want to use collections Mapping instead of dict as per the ABC PEP   def value list x       if isinstance x  dict           return list set x values         elif isinstance x  basestring           return  x      else          return None

User · Answer

built-in types in Python have built in names    gt  gt  gt  s    hallo   gt  gt  gt  type s  is str True  gt  gt  gt  s       gt  gt  gt  type s  is dict True   btw note the is operator  However  type checking  if you want to call it that  is usually done by wrapping a type-specific test in a try-except clause  as it s not so much the type of the variable that s important  but whether you can do a certain something with it or not

User · Answer

That should work - so no  there is nothing wrong with your code  However  it could also be done with a dict    type str     do something with a string   type dict     do something with a dict  get type x   errorhandler      A bit more concise and pythonic wouldn t you say     Edit   Heeding Avisser s advice  the code also works like this  and looks nicer    str  do something with a string   dict  do something with a dict  get type x   errorhandler

User · Answer

I think it might be preferred to actually do  if isinstance x  str       do something with a string x  elif isinstance x  dict       do somethting with a dict x  else      raise ValueError   2 Alternate forms  depending on your code one or the other is probably considered better than that even   One is to not look before you leap  try    one  two   tupleOrValue except TypeError    one   tupleOrValue   two   None   The other approach is from Guido and is a form of function overloading which leaves your code more open ended   http   www artima com weblogs viewpost jsp thread 155514

User · Answer

built-in types in Python have built in names    gt  gt  gt  s    hallo   gt  gt  gt  type s  is str True  gt  gt  gt  s       gt  gt  gt  type s  is dict True   btw note the is operator  However  type checking  if you want to call it that  is usually done by wrapping a type-specific test in a try-except clause  as it s not so much the type of the variable that s important  but whether you can do a certain something with it or not

User · Answer

sigh   No  typechecking arguments in python is not necessary  It is never  necessary   If your code accepts either a string or a dict object  your design is broken   That comes from the fact that if you don t know already the type of an object in your own program  then you re doing something wrong already   Typechecking hurts code reuse and reduces performance  Having a function that performs different things depending on the type of the object passed is  bug-prone and has a behavior harder to understand and maintain   You have the following saner options   1  Make a function unique values that converts dicts in unique lists of values   def unique values some dict       return list set some dict values       Make your function assume the argument passed is always a list  That way  if you need to pass a string to the function  you just do   myfunction  some string     If you need to pass it a dict  you do   myfunction unique values some dict     That s your best option  it is clean  easy to understand and maintain  Anyone reading the code immediatelly understands what is happening  and you don t have to typecheck   2  Make two functions  one that accepts lists of strings and one that accepts  dicts  You can make one call the other internally  in the most convenient  way  myfunction dict can create a list of strings and call myfunction list    In any case  don t typecheck  It is completely unnecessary and has only  downsides  Refactor your code instead in a way you don t need to typecheck   You only get benefits in doing so  both in short and long run

User · Answer

What happens if somebody passes a unicode string to your function  Or a class derived from dict  Or a class implementing a dict-like interface  Following code covers first two cases  If you are using Python 2 6 you might want to use collections Mapping instead of dict as per the ABC PEP   def value list x       if isinstance x  dict           return list set x values         elif isinstance x  basestring           return  x      else          return None

User · Answer

That should work - so no  there is nothing wrong with your code  However  it could also be done with a dict    type str     do something with a string   type dict     do something with a dict  get type x   errorhandler      A bit more concise and pythonic wouldn t you say     Edit   Heeding Avisser s advice  the code also works like this  and looks nicer    str  do something with a string   dict  do something with a dict  get type x   errorhandler

[python] What is the best (idiomatic) way to check the type of a Python variable?

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