What is getattr exactly and how do I use it

Question

I ve recently read about the getattr   function  The problem is that I still can t grasp the idea of its usage  The only thing I understand about getattr   is that getattr li   pop   is the same as calling li pop   I didn t understand when the book mentioned how you use it to get a reference to a function without knowing its name until run-time  Maybe this is me being a noob in programming  in general  Could anyone shed some light on the subject  When and how do I use this exactly

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getattr object   x   is completely equivalent to object x    There are only two cases where getattr can be useful    you can t write object x  because you don t know in advance which attribute you want  it comes from a string    Very useful for meta-programming  you want to provide a default value  object y will raise an AttributeError if there s no y  But getattr object   y   5  will return 5

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I think this example is self explanatory  It runs the method of first parameter  whose name is given in the second parameter   class MyClass     def   init   self         pass    def MyMethod self         print  Method ran      Create an object object   MyClass     Get all the methods of a class method list    func for func in dir MyClass  if callable getattr MyClass  func      You can use any of the methods in method list    MyMethod  is the one we want to use right now    This is the same as running  object MyMethod    getattr object  MyMethod

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getattr  class hithere         def french self           print  bonjour       def english self           print  hello       def german self           print  hallo       def czech self           print  ahoj       def noidea self           print  unknown language    def dispatch language       try          getattr hithere   language        except          getattr hithere    noidea               note  do better error handling than this  dispatch  french   dispatch  english   dispatch  german   dispatch  czech   dispatch  spanish

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setattr    We use setattr to add an attribute to our class instance  We pass the class instance  the attribute name  and the value    getattr    With getattr we retrive these values  For example  Employee   type  Employee    object    dict     employee   Employee      Set salary to 1000 setattr employee  salary   1000      Get the Salary value   getattr employee   salary    print value

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I have tried in Python2 7 17  Some of the fellow folks already answered  However I have tried to call getattr obj   set value   and this didn t execute the set value method  So i changed to getattr obj   set value     --  This helps to invoke the same   Example Code   Example 1       class GETATT VERIFY           name    siva         def   init   self              print  Ok         def set value self              self value    myself             print  oooh      obj   GETATT VERIFY       print getattr GETATT VERIFY   name       getattr obj   set value         print obj value

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Other than all the amazing answers here  there is a way to use getattr to save copious lines of code and keeping it snug  This thought came following the dreadful representation of code that sometimes might be a necessity   Scenario  Suppose your directory structure is as follows   - superheroes py - properties py   And  you have functions for getting information about Thor  Iron Man  Doctor Strange in superheroes py  You very smartly write down the properties of all of them in properties py in a compact dict and then access them   properties py  thor          about    Asgardian god of thunder        weapon    Mjolnir        powers     invulnerability    keen senses    vortex breath      and many more   iron man          about    A wealthy American business magnate  playboy  and ingenious scientist        weapon    Armor        powers     intellect    armor suit    interface with wireless connections    money      doctor strange          about     primary protector of Earth against magical and mystical threats        weapon    Magic        powers     magic    intellect    martial arts        Now  let s say you want to return capabilities of each of them on demand in superheroes py  So  there are functions like  from  properties import thor  iron man  doctor strange   def get thor weapon        return thor  weapon     def get iron man bio        return iron man  about     def get thor powers        return thor  powers        and more functions returning different values based on the keys and superhero   With the help of getattr  you could do something like   from   import properties   def get superhero weapon hero       superhero   getattr properties  hero      return superhero  weapon     def get superhero powers hero       superhero   getattr properties  hero      return superhero  powers     You considerably reduced the number of lines of code  functions and repetition   Oh and of course  if you have bad names like properties of thor for variables   they can be made and accessed by simply doing  def get superhero weapon hero       superhero    properties of     format hero      all properties   getattr properties  superhero      return all properties  weapon     NOTE  For this particular problem  there can be smarter ways to deal with the situation  but the idea is to give an insight about using getattr in right places to write cleaner code

User · Answer

Objects in Python can have attributes -- data attributes and functions to work with those  methods   Actually  every object has built-in attributes   For example you have an object person  that has several attributes  name  gender  etc   You access these attributes  be it methods or data objects  usually writing  person name  person gender  person the method    etc   But what if you don t know the attribute s name at the time you write the program  For example you have attribute s name stored in a variable called attr name   if  attr name    gender    then  instead of writing  gender   person gender   you can write  gender   getattr person  attr name    Some practice   Python 3 4 0  default  Apr 11 2014  13 05 11    gt  gt  gt  class Person            name    Victor          def say self  what               print self name  what        gt  gt  gt  getattr Person   name    Victor   gt  gt  gt  attr name    name   gt  gt  gt  person   Person    gt  gt  gt  getattr person  attr name   Victor   gt  gt  gt  getattr person   say    Hello   Victor Hello   getattr will raise AttributeError if attribute with the given name does not exist in the object    gt  gt  gt  getattr person   age   Traceback  most recent call last     File   lt stdin gt    line 1  in  lt module gt  AttributeError   Person  object has no attribute  age    But you can pass a default value as the third argument  which will be returned if such attribute does not exist    gt  gt  gt  getattr person   age   0  0   You can use getattr along with dir to iterate over all attribute names and get their values    gt  gt  gt  dir 1000      abs        add             trunc        xor      bit length    conjugate    denominator    from bytes    imag    numerator    real    to bytes     gt  gt  gt  obj   1000  gt  gt  gt  for attr name in dir obj           attr value   getattr obj  attr name          print attr name  attr value  callable attr value          abs    lt method-wrapper    abs    of int object at 0x7f4e927c2f90 gt  True     bit length  lt built-in method bit length of int object at 0x7f4e927c2f90 gt  True       gt  gt  gt  getattr 1000   bit length     10   A practical use for this would be to find all methods whose names start with test and call them   Similar to getattr there is setattr which allows you to set an attribute of an object having its name    gt  gt  gt  setattr person   name    Andrew    gt  gt  gt  person name    accessing instance attribute  Andrew   gt  gt  gt  Person name    accessing class attribute  Victor   gt  gt  gt

User · Answer

I sometimes use getattr     to lazily initialise attributes of secondary importance just before they are used in the code    Compare the following   class Graph object       def   init   self           self n calls to plot   0                A lot of code here               def plot self           self n calls to plot    1   To this   class Graph object       def plot self           self n calls to plot   1   getattr self   n calls to plot   0    The advantage of the second way is that n calls to plot only appears around the place in the code where it is used  This is good for readability  because  1  you can immediately see what value it starts with when reading how it s used   2  it doesn t introduce a distraction into the   init       method  which ideally should be about the conceptual state of the class  rather than some utility counter that is only used by one of the function s methods for technical reasons  such as optimisation  and has nothing to do with the meaning of the object

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It is also clarifying from https   www programiz com python-programming methods built-in getattr   class Person      age   23     name    Adam   person   Person   print  The age is    getattr person   age    print  The age is    person age    The age is  23  The age is  23  class Person      age   23     name    Adam   person   Person      when default value is provided print  The sex is    getattr person   sex    Male       when no default value is provided print  The sex is    getattr person   sex      The sex is  Male  AttributeError   Person  object has no attribute  sex

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For me  getattr is easiest to explain this way   It allows you to call methods based on the contents of a string instead of typing the method name   For example  you cannot do this   obj   MyObject   for x in   foo    bar        obj x     because x is not of the type builtin  but str  However  you CAN do this   obj   MyObject   for x in   foo    bar        getattr obj  x      It allows you to dynamically connect with objects based on your input  I ve found it useful when dealing with custom objects and modules

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Quite frequently when I am creating an XML file from data stored in a class I would frequently receive errors if the attribute didn t exist or was of type None   In this case  my issue wasn t not knowing what the attribute name was  as stated in your question  but rather was data ever stored in that attribute   class Pet      def   init   self           self hair   None         self color   None   If I used hasattr to do this  it would return True even if the attribute value was of type None and this would cause my ElementTree set command to fail   hasattr temp   hair    gt  gt True   If the attribute value was of type None  getattr would also return it which would cause my ElementTree set command to fail     c   getattr temp   hair   type c   gt  gt  NoneType   I use the following method to take care of these cases now   def getRealAttr class obj  class attr  default            temp   getattr class obj  class attr  default      if temp is None          temp   default     elif type temp     str          temp   str temp      return temp   This is when and how I use getattr

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Another use of getattr   in implementing a switch statement in Python  It uses both reflection to get the case type   import sys  class SwitchStatement object           a class to implement switch statement and a way to show how to use gettattr in Pythion         def case 1 self           return  value for case 1       def case 2 self           return  value for case 2       def case 3 self           return  value for case 3       def case 4 self           return  value for case 4       def case value self  case type 1              This is the main dispatchmethod  that uses gettattr            case method    case     str case type            fetch the relevant method name           Get the method from  self   Default to a lambda          method   getattr self  case method  lambda   Invalid case type             Call the method as we return it         return method    def main         switch   SwitchStatement       print swtich case value     if   name         main         main int sys argv 1

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Here s a quick and dirty example of how a class could fire different versions of a save method depending on which operating system it s being executed on using getattr     import os  class Log object       def   init   self           self os   os name     def   getattr   self  name               look for a  save  attribute  or just            return whatever attribute was specified             if name     save               try                    try to dynamically return a save                    method appropriate for the user s system                 return getattr self  self os              except                    bail and try to return                    a default save method                 return getattr self    save           else              return getattr self  name         each of these methods could have save logic specific to        the system on which the script is executed     def posix self   print  saving on a posix machine      def nt self   print  saving on an nt machine      def os2 self   print  saving on an os2 machine      def ce self   print  saving on a ce machine      def java self   print  saving on a java machine      def riscos self   print  saving on a riscos machine      def  save self   print  saving on an unknown operating system       def which os self   print os name   Now let s use this class in an example   logger   Log      Now you can do one of two things  save func   logger save   and execute it  or pass it along    somewhere else as 1st class  save func      or you can just call it directly  logger save      other attributes will hit the else    statement and still work as expected logger which os

User · Answer

A pretty common use case for getattr is mapping data to functions     For instance  in a web framework like Django or Pylons  getattr makes it straightforward to map a web request s URL to the function that s going to handle it   If you look under the hood of Pylons s routing  for instance  you ll see that  by default  at least  it chops up a request s URL  like   http   www example com customers list   into  customers  and  list    Then it searches for a controller class named CustomerController   Assuming it finds the class  it creates an instance of the class and then uses getattr to get its list method   It then calls that method  passing it the request as an argument   Once you grasp this idea  it becomes really easy to extend the functionality of a web application   just add new methods to the controller classes  and then create links in your pages that use the appropriate URLs for those methods   All of this is made possible by getattr

[python] What is getattr() exactly and how do I use it?

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