How to know if an object has an attribute in Python

Question

Is there a way in Python to determine if an object has some attribute   For example    gt  gt  gt  a   SomeClass    gt  gt  gt  a someProperty   value  gt  gt  gt  a property Traceback  most recent call last     File   lt stdin gt    line 1  in  lt module gt  AttributeError  SomeClass instance has no attribute  property    How can you tell if a has the attribute property before using it

User · Answer

Here s a very intuitive approach    if  property  in dir a       a property

User · Answer

hasattr   is the right answer  What I want to add is that hasattr   can also be used well in conjunction with assert  to avoid unnecessary if statements and make the code more readable    assert hasattr a   property     object lacks property     As stated in another answer on SO  Asserts should be used to test conditions that should never happen  The purpose is to crash early in the case of a corrupt program state

User · Answer

Try hasattr     if hasattr a   property        a property   EDIT  See zweiterlinde s answer below  who offers good advice about asking forgiveness  A very pythonic approach    The general practice in python is that  if the property is likely to be there most of the time  simply call it and either let the exception propagate  or trap it with a try except block  This will likely be faster than hasattr  If the property is likely to not be there most of the time  or you re not sure  using hasattr will probably be faster than repeatedly falling into an exception block

User · Answer

According to pydoc  hasattr obj  prop  simply calls getattr obj  prop  and catches exceptions  So  it is just as valid to wrap the attribute access with a try statement and catch AttributeError as it is to use hasattr   beforehand   a   SomeClass   try      return a fake prop except AttributeError      return default value

User · Answer

You can check whether object contains attribute by using hasattr builtin method   For an instance if your object is a and you want to check for attribute stuff   gt  gt  gt  class a          stuff    something        gt  gt  gt  hasattr a  stuff   True  gt  gt  gt  hasattr a  other stuff   False   The method signature itself is hasattr object  name  - gt  bool which mean if object has attribute which is passed to second argument in hasattr than it gives boolean True or False according to the presence of name attribute in object

User · Answer

This is super simple  just use dir object  This will return a list of every available function and attribute of the object

User · Answer

I think what you are looking for is hasattr  However  I d recommend something like this if you want to detect python properties-  try      getattr someObject   someProperty            except AttributeError      print  Doesn t exist  else     print  Exists    The disadvantage here is that attribute errors in the properties   get   code are also caught   Otherwise  do-  if hasattr someObject   someProp         Access someProp  set someProp     pass   Docs http   docs python org library functions html Warning  The reason for my recommendation is that hasattr doesn t detect properties  Link http   mail python org pipermail python-dev 2005-December 058498 html

User · Answer

I would like to suggest avoid this   try      doStuff a property  except AttributeError      otherStuff     The user  jpalecek mentioned it  If an AttributeError occurs inside doStuff    you are lost   Maybe this approach is better   try      val   a property except AttributeError      otherStuff   else      doStuff val

User · Answer

Depending on the situation you can check with isinstance what kind of object you have  and then use the corresponding attributes  With the introduction of abstract base classes in Python 2 6 3 0 this approach has also become much more powerful  basically ABCs allow for a more sophisticated way of duck typing    One situation were this is useful would be if two different objects have an attribute with the same name  but with different meaning  Using only hasattr might then lead to strange errors   One nice example is the distinction between iterators and iterables  see this question   The   iter   methods in an iterator and an iterable have the same name but are semantically quite different  So hasattr is useless  but isinstance together with ABC s provides a clean solution   However  I agree that in most situations the hasattr approach  described in other answers  is the most appropriate solution

User · Answer

Another possible option  but it depends if what you mean by before   undefined   object    class Widget       def   init   self           self bar   1      def zoom self           print  zoom     a   Widget    bar   getattr a   bar   undefined  if bar is not undefined      print  bar  s     bar    foo   getattr a   foo   undefined  if foo is not undefined      print  foo  s     foo    zoom   getattr a   zoom   undefined  if zoom is not undefined      zoom     output   bar 1 zoom    This allows you to even check for None-valued attributes   But   Be very careful you don t accidentally instantiate and compare undefined multiple places because the is will never work in that case   Update   because of what I was warning about in the above paragraph  having multiple undefineds that never match  I have recently slightly modified this pattern   undefined   NotImplemented  NotImplemented  not to be confused with NotImplementedError  is a built-in   it semi-matches the intent of a JS undefined and you can reuse its definition everywhere and it will always match   The drawbacks is that it is  truthy  in booleans and it can look weird in logs and stack traces  but you quickly get over it when you know it only appears in this context

User · Answer

You can use hasattr   or catch AttributeError  but if you really just want the value of the attribute with a default if it isn t there  the best option is just to use getattr     getattr a   property    default value

User · Answer

As Jarret Hardie answered  hasattr will do the trick   I would like to add  though  that many in the Python community recommend a strategy of  easier to ask for forgiveness than permission   EAFP  rather than  look before you leap   LBYL    See these references   EAFP vs LBYL  was Re  A little disappointed so far  EAFP vs  LBYL  Code Like a Pythonista  Idiomatic Python  ie   try      doStuff a property  except AttributeError      otherStuff         is preferred to   if hasattr a   property        doStuff a property  else      otherStuff

User · Answer

Hope you expecting hasattr    but try to avoid hasattr   and please prefer getattr    getattr   is faster than hasattr    using hasattr      if hasattr a   property         print a property   same here i am using getattr to get property if there is no property it return none     property   getattr a  property  None      if property          print property

User · Answer

EDIT This approach has serious limitation  It should work if the object is an iterable one  Please check the comments below   If you are using Python 3 6 or higher like me there is a convenient alternative to check whether an object has a particular attribute   if  attr1  in obj1      print  attr1       format obj1  attr1       However  I m not sure which is the best approach right now  using hasattr    using getattr   or using in  Comments are welcome

[python] How to know if an object has an attribute in Python

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