cpython: 8e0b7393e921 (original) (raw)

Mercurial > cpython

changeset 91350:8e0b7393e921 2.7

Issue #11974: Add tutorial section on class and instance variables (Based on a patch from Renee Chu.) [#11974]

Raymond Hettinger python@rcn.com
date	Mon, 23 Jun 2014 18:03:21 -0700
parents	e28004fb30c0
children	8d963c7db507
files	Doc/tutorial/classes.rst Misc/ACKS
diffstat	2 files changed, 72 insertions(+), 0 deletions(-)[+] [-] Doc/tutorial/classes.rst 71 Misc/ACKS 1

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line diff

--- a/Doc/tutorial/classes.rst +++ b/Doc/tutorial/classes.rst @@ -337,6 +337,77 @@ object and the argument list, and the fu argument list. +.. _tut-class-and-instance-variables: + +Class and Instance Variables +---------------------------- + +Generally speaking, instance variables are for data unique to each instance +and class variables are for attributes and methods shared by all instances +of the class:: +

class Dog:

   kind = 'canine'         # class variable shared by all instances[](#l1.18)

```
   def __init__(self, name):[](#l1.20)
```

       self.name = name    # instance variable unique to each instance[](#l1.21)

d = Dog('Fido')
e = Dog('Buddy')
d.kind # shared by all dogs
'canine'
e.kind # shared by all dogs
'canine'
d.name # unique to d
'Fido'
e.name # unique to e
'Buddy'

+ +As discussed in :ref:tut-object, shared data can have possibly surprising +effects with involving :term:mutable objects such as lists and dictionaries. +For example, the tricks list in the following code should not be used as a +class variable because just a single list would be shared by all Dog +instances:: +

class Dog:

   tricks = []             # mistaken use of a class variable[](#l1.42)

```
   def __init__(self, name):[](#l1.44)
```
```
       self.name = name[](#l1.45)
```

   def add_trick(self, trick):[](#l1.47)

       self.tricks.append(trick)[](#l1.48)

d = Dog('Fido')
e = Dog('Buddy')
d.add_trick('roll over')
e.add_trick('play dead')
d.tricks # unexpectedly shared by all dogs
['roll over', 'play dead']

+ +Correct design of the class should use an instance variable instead:: +

class Dog:

```
   def __init__(self, name):[](#l1.61)
```
```
       self.name = name[](#l1.62)
```

       self.tricks = []    # creates a new empty list for each dog[](#l1.63)

   def add_trick(self, trick):[](#l1.65)

       self.tricks.append(trick)[](#l1.66)

d = Dog('Fido')
e = Dog('Buddy')
d.add_trick('roll over')
e.add_trick('play dead')
d.tricks
['roll over']
e.tricks
['play dead']

+ + .. _tut-remarks: Random Remarks

--- a/Misc/ACKS +++ b/Misc/ACKS @@ -235,6 +235,7 @@ Adal Chiriliuc Matt Chisholm Anders Chrigström Tom Christiansen +Renee Chu Vadim Chugunov Mauro Cicognini David Cinege