Python Variable Declaration

Question

Learning Python  and has some basic doubts   1 I have seen variable declaration  path here  as   class writer      path        sometimes  no explicit declaration but initialize through   init      def   init   self  name       self name   name   I understand the purpose of   init    but is it advisable to declare variable in any other functions   2 How can I create variable to hold a custom type   class writer      path        string value     customObj

User · Answer

For scoping purpose  I use   custom object   None

User · Answer

Variables have scope  so yes it is appropriate to have variables that are specific to your function  You don t always have to be explicit about their definition  usually you can just use them  Only if you want to do something specific to the type of the variable  like append for a list  do you need to define them before you start using them  Typical example of this   list      for i in stuff    list append i    By the way  this is not really a good way to setup the list  It would be better to say   list    i for i in stuff    list comprehension      but I digress   Your other question  The custom object should be a class itself   class CustomObject      always capitalize the class name   this is not syntax  just style    pass customObj   CustomObject

User · Answer

There s no need to declare new variables in Python  If we re talking about variables in functions or modules  no declaration is needed  Just assign a value to a name where you need it  mymagic    Magic   Variables in Python can hold values of any type  and you can t restrict that   Your question specifically asks about classes  objects and instance variables though  The idiomatic way to create instance variables is in the   init   method and nowhere else     while you could create new instance variables in other methods  or even in unrelated code  it s just a bad idea  It ll make your code hard to reason about or to maintain   So for example   class Thing object        def   init   self  magic           self magic   magic   Easy  Now instances of this class have a magic attribute   thingo   Thing  More magic     thingo magic is now  More magic    Creating variables in the namespace of the class itself leads to different behaviour altogether  It is functionally different  and you should only do it if you have a specific reason to  For example   class Thing object        magic    Magic       def   init   self           pass   Now try   thingo   Thing   Thing magic   1   thingo magic is now 1   Or   class Thing object        magic     More    magic        def   init   self           pass  thing1   Thing   thing2   Thing   thing1 magic append  here     thing1 magic AND thing2 magic is now   More    magic    here     This is because the namespace of the class itself is different to the namespace of the objects created from it  I ll leave it to you to research that a bit more   The take-home message is that idiomatic Python is to  a  initialise object attributes in your   init   method  and  b  document the behaviour of your class as needed  You don t need to go to the trouble of full-blown Sphinx-level documentation for everything you ever write  but at least some comments about whatever details you or someone else might need to pick it up

User · Answer

This might be 6 years late  but in Python 3 5 and above  you declare a variable type like this   variable name  type name   or this   variable name   type  shinyType   So in your case if you have a CustomObject class defined   you can do   customObj  CustomObject   See this or that for more info

User · Answer

Okay  first things first   There is no such thing as  variable declaration  or  variable initialization  in Python   There is simply what we call  assignment   but should probably just call  naming    Assignment means  this name on the left-hand side now refers to the result of evaluating the right-hand side  regardless of what it referred to before  if anything     foo    bar    the name  foo  is now a name for the string  bar  foo   2   3   the name  foo  stops being a name for the string  bar     and starts being a name for the integer 6  resulting from the multiplication   As such  Python s names  a better term than  variables   arguably  don t have associated types  the values do  You can re-apply the same name to anything regardless of its type  but the thing still has behaviour that s dependent upon its type  The name is simply a way to refer to the value  object   This answers your second question  You don t create variables to hold a custom type  You don t create variables to hold any particular type  You don t  create  variables at all  You give names to objects   Second point  Python follows a very simple rule when it comes to classes  that is actually much more consistent than what languages like Java  C   and C  do  everything declared inside the class block is part of the class  So  functions  def  written here are methods  i e  part of the class object  not stored on a per-instance basis   just like in Java  C   and C   but other names here are also part of the class  Again  the names are just names  and they don t have associated types  and functions are objects too in Python  Thus   class Example      data   42     def method self   pass   Classes are objects too  in Python   So now we have created an object named Example  which represents the class of all things that are Examples  This object has two user-supplied attributes  In C     members   in C    fields or properties or methods   in Java   fields or methods    One of them is named data  and it stores the integer value 42  The other is named method  and it stores a function object   There are several more attributes that Python adds automatically    These attributes still aren t really part of the object  though  Fundamentally  an object is just a bundle of more names  the attribute names   until you get down to things that can t be divided up any more  Thus  values can be shared between different instances of a class  or even between objects of different classes  if you deliberately set that up   Let s create an instance   x   Example     Now we have a separate object named x  which is an instance of Example  The data and method are not actually part of the object  but we can still look them up via x because of some magic that Python does behind the scenes  When we look up method  in particular  we will instead get a  bound method   when we call it  x gets passed automatically as the self parameter  which cannot happen if we look up Example method directly    What happens when we try to use x data   When we examine it  it s looked up in the object first  If it s not found in the object  Python looks in the class   However  when we assign to x data  Python will create an attribute on the object  It will not replace the class  attribute   This allows us to do object initialization  Python will automatically call the class    init   method on new instances when they are created  if present  In this method  we can simply assign to attributes to set initial values for that attribute on each object   class Example      name    Ignored      def   init   self  name           self name   name       rest as before   Now we must specify a name when we create an Example  and each instance has its own name  Python will ignore the class attribute Example name whenever we look up the  name of an instance  because the instance s attribute will be found first   One last caveat  modification  mutation  and assignment are different things   In Python  strings are immutable  They cannot be modified  When you do   a    hi   b   a a     mom    You do not change the original  hi   string  That is impossible in Python  Instead  you create a new string  hi mom   and cause a to stop being a name for  hi    and start being a name for  hi mom  instead  We made b a name for  hi   as well  and after re-applying the a name  b is still a name for  hi    because  hi   still exists and has not been changed   But lists can be changed   a    1  2  3  b   a a     4    Now b is  1  2  3  4  as well  because we made b a name for the same thing that a named  and then we changed that thing  We did not create a new list for a to name  because Python simply treats    differently for lists   This matters for objects because if you had a list as a class attribute  and used an instance to modify the list  then the change would be  seen  in all other instances  This is because  a  the data is actually part of the class object  and not any instance object   b  because you were modifying the list and not doing a simple assignment  you did not create a new instance attribute hiding the class attribute

[python] Python Variable Declaration

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