[Python-checkins] cpython (2.7): Issue #11974: Add tutorial section on class and instance variables
raymond.hettinger
python-checkins at python.org
Tue Jun 24 03:03:30 CEST 2014
http://hg.python.org/cpython/rev/8e0b7393e921
changeset: 91350:8e0b7393e921
branch: 2.7
parent: 91345:e28004fb30c0
user: Raymond Hettinger <python at rcn.com>
date: Mon Jun 23 18:03:21 2014 -0700
summary:
Issue #11974: Add tutorial section on class and instance variables
(Based on a patch from Renee Chu.)
files:
Doc/tutorial/classes.rst | 71 ++++++++++++++++++++++++++++
Misc/ACKS | 1 +
2 files changed, 72 insertions(+), 0 deletions(-)
diff --git a/Doc/tutorial/classes.rst b/Doc/tutorial/classes.rst
--- a/Doc/tutorial/classes.rst
+++ b/Doc/tutorial/classes.rst
@@ -337,6 +337,77 @@
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
+
+ def __init__(self, name):
+ self.name = name # instance variable unique to each instance
+
+ >>> 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
+
+ def __init__(self, name):
+ self.name = name
+
+ def add_trick(self, trick):
+ self.tricks.append(trick)
+
+ >>> 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):
+ self.name = name
+ self.tricks = [] # creates a new empty list for each dog
+
+ def add_trick(self, trick):
+ self.tricks.append(trick)
+
+ >>> 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
diff --git a/Misc/ACKS b/Misc/ACKS
--- a/Misc/ACKS
+++ b/Misc/ACKS
@@ -235,6 +235,7 @@
Matt Chisholm
Anders Chrigström
Tom Christiansen
+Renee Chu
Vadim Chugunov
Mauro Cicognini
David Cinege
--
Repository URL: http://hg.python.org/cpython
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