numInstances = 0
def __init__(self):
Spam.numInstances = Spam.numInstances + 1
def printNumInstances():
print("Number of instances created: %s" % Spam.numInstances)
# py -2
from spam import Spam
a = Spam() # 2.X에서는 언바운드 클래스 메서드를 호출할 수 없음
b = Spam() # 기본적으로 메서드는 self 객체를 기대함
c = Spam()
Spam.printNumInstances()
# TypeError: unbound method printNumInstances() must be called with Spam instance as first argument (got nothing instead)
a.printNumInstances()
# TypeError: printNumInstances() takes no arguments (1 given)
# py -3
from spam import Spam
a = Spam() # 3.X에서는 클래스에서 함수를 호출할 수 있음
b = Spam() # 인스턴스를 통한 호출은 여전히 self를 전달함
c = Spam()
Spam.printNumInstances() # 3.X에서는 다름
# Number of instances created: 3
a.printNumInstances()
# TypeError: printNumInstances() takes 0 positional arguments but 1 was given
Spam.numInstances = Spam.numInstances + 1
# py -3
import spam
a = spam.Spam()
b = spam.Spam()
c = spam.Spam()
spam.printNumInstances() # 하지만 함수는 극단적으로 제거될 수 있음
# Number of instances created: 3 # 그리고 상속에 의해 변경될 수 없음
spam.Spam.numInstances
# 3
class Spam:
numInstances = 0
def __init__(self):
Spam.numInstances = Spam.numInstances + 1
def printNumInstances(self):
print("Number of instances created: %s" % Spam.numInstances)
# py -3
from spam import Spam
a, b, c = Spam(), Spam(), Spam()
a.printNumInstances()
# Number of instances created: 3
Spam.printNumInstances(a)
# Number of instances created: 3
Spam().printNumInstances() # 하지만 카운터를 가져오는 것은 카운터를 변경시킴!
# Number of instances created: 4
def printNumInstances(): # Override a static method
print("Extra stuff...") # But call back to original
Spam.printNumInstances()
def printNumInstances(cls): # Override a class method
print("Extra stuff...", cls) # But call back to original
Spam.printNumInstances()
>>> class E:
... __slots__ = ['c', 'd'] # Superclass has slots
...
>>> class D(E):
... __slots__ = ['a', '__dict__'] # So does its subclass
...
>>> X = D()
>>> X.a = 1; X.b = 2; X.c = 3 # The instance is the union
>>> X.a, X.c
(1, 3)
>>> E.__slots__ # But slots are not concatenated
['c', 'd']
>>> D.__slots__
['a', '__dict__']
>>> X.__slots__ # Instance inherits *lowest* __slots__
['a', '__dict__']
>>> X.__dict__ # And has its own an attr dict
{'b': 2}
>>> for attr in list(getattr(X, '__dict__', [])) + getattr(X, '__slots__', []):
... print(attr, '=>', getattr(X, attr))
...
b => 2 # Superclass slots missed!
a => 1
__dict__ => {'b': 2}
>>> dir(X) # dir() includes all slot names
[...many names omitted... 'a', 'b', 'c', 'd']
>>> class classic:
... def __getattr__(self, name):
... if name == 'age':
... return 40
... else:
... raise AttributeError
...
>>> x = classic()
>>> x.age # Runs __getattr__
40
>>> x.name # Runs __getattr__
AttributeError
>>> class newprops(object):
... def getage(self):
... return 40
... age = property(getage, None, None, None) # get, set, del, docs
...
>>> x = newprops()
>>> x.age # Runs getage
40
>>> x.name # Normal fetch
AttributeError: newprops instance has no attribute 'name'
>>> class newprops(object):
... def getage(self):
... return 40
... def setage(self, value):
... print('set age:', value)
... self._age = value
... age = property(getage, setage, None, None)
...
>>> x = newprops()
>>> x.age # Runs getage
40
>>> x.age = 42 # Runs setage
set age: 42
>>> x._age # Normal fetch; no getage call
42
>>> x.job = 'trainer' # Normal assign; no setage call
>>> x.job # Normal fetch; no getage call
'trainer'
>>> class classic:
... def __getattr__(self, name): # On undefined reference
... if name == 'age':
... return 40
... else:
... raise AttributeError
... def __setattr__(self, name, value): # On all assignments
... print('set:', name, value)
... if name == 'age':
... self.__dict__['_age'] = value
... else:
... self.__dict__[name] = value
...
>>> x = classic()
>>> x.age # Runs __getattr__
40
>>> x.age = 41 # Runs __setattr__
set: age 41
>>> x._age # Defined: no __getattr__ call
41
>>> x.job = 'trainer' # Runs __setattr__ again
>>> x.job # Defined: no __getattr__ call
### file: spam.py
class Spam:
numInstances = 0
def __init__(self):
Spam.numInstances = Spam.numInstances + 1
def printNumInstances():
print("Number of instances created: ", Spam.numInstances)
C:\misc> c:\python26\python
>>> from spam import Spam
>>> a = Spam() # Cannot call unbound class methods in 2.6
>>> b = Spam() # Methods expect a self object by default
>>> c = Spam()
>>> Spam.printNumInstances()
TypeError: unbound method printNumInstances() must be called with Spam instance
as first argument (got nothing instead)
>>> a.printNumInstances()
TypeError: printNumInstances() takes no arguments (1 given)
C:\misc> c:\python30\python
>>> from spam import Spam
>>> a = Spam() # Can call functions in class in 3.0
>>> b = Spam() # Calls through instances still pass a self
>>> c = Spam()
>>> Spam.printNumInstances() # Differs in 3.0
Number of instances created: 3
>>> a.printNumInstances()
TypeError: printNumInstances() takes no arguments (1 given)
Spam.printNumInstances() # Fails in 2.6, works in 3.0
instance.printNumInstances() # Fails in both 2.6 and 3.0
### file spam.py (changed)
def printNumInstances():
print("Number of instances created: ", Spam.numInstances)
class Spam:
numInstances = 0
def __init__(self):
Spam.numInstances = Spam.numInstances + 1
>>> import spam
>>> a = spam.Spam()
>>> b = spam.Spam()
>>> c = spam.Spam()
>>> spam.printNumInstances() # But function may be too far removed
Number of instances created: 3 # And cannot be changed via inheritance
>>> spam.Spam.numInstances
3
class Spam:
numInstances = 0
def __init__(self):
Spam.numInstances = Spam.numInstances + 1
def printNumInstances(self):
print("Number of instances created: ", Spam.numInstances)
>>> from spam import Spam
>>> a, b, c = Spam(), Spam(), Spam()
>>> a.printNumInstances()
Number of instances created: 3
>>> Spam.printNumInstances(a)
Number of instances created: 3
>>> Spam().printNumInstances() # But fetching counter changes counter!
Number of instances created: 4
class Methods:
def imeth(self, x): # Normal instance method: passed a self
print(self, x)
def smeth(x): # Static: no instance passed
print(x)
def cmeth(cls, x): # Class: gets class, not instance
print(cls, x)
smeth = staticmethod(smeth) # Make smeth a static method
cmeth = classmethod(cmeth) # Make cmeth a class method
>>> obj = Methods() # Make an instance
>>> obj.imeth(1) # Normal method, call through instance
<__main__.Methods object...> 1 # Becomes imeth(obj, 1)
>>> Methods.imeth(obj, 2) # Normal method, call through class
<__main__.Methods object...> 2 # Instance passed explicitly
>>> Methods.smeth(3) # Static method, call through class
3 # No instance passed or expected
>>> obj.smeth(4) # Static method, call through instance
4 # Instance not passed
>>> Methods.cmeth(5) # Class method, call through class
<class '__main__.Methods'> 5 # Becomes cmeth(Methods, 5)
>>> obj.cmeth(6) # Class method, call through instance
<class '__main__.Methods'> 6 # Becomes cmeth(Methods, 6)
class Spam:
numInstances = 0 # Use static method for class data
def __init__(self):
Spam.numInstances += 1
def printNumInstances():
print("Number of instances:", Spam.numInstances)
printNumInstances = staticmethod(printNumInstances)
>>> a = Spam()
>>> b = Spam()
>>> c = Spam()
>>> Spam.printNumInstances() # Call as simple function
Number of instances: 3
>>> a.printNumInstances() # Instance argument not passed
Number of instances: 3
class Sub(Spam):
def printNumInstances(): # Override a static method
print("Extra stuff...") # But call back to original
Spam.printNumInstances()
printNumInstances = staticmethod(printNumInstances)
>>> a = Sub()
>>> b = Sub()
>>> a.printNumInstances() # Call from subclass instance
Extra stuff...
Number of instances: 2
>>> Sub.printNumInstances() # Call from subclass itself
Extra stuff...
Number of instances: 2
>>> Spam.printNumInstances()
Number of instances: 2
>>> class Other(Spam): pass # Inherit static method verbatim
>>> c = Other()
>>> c.printNumInstances()
Number of instances: 3
class Spam:
numInstances = 0 # Use class method instead of static
def __init__(self):
Spam.numInstances += 1
def printNumInstances(cls):
print("Number of instances:", cls.numInstances)
printNumInstances = classmethod(printNumInstances)
>>> a, b = Spam(), Spam()
>>> a.printNumInstances() # Passes class to first argument
Number of instances: 2
>>> Spam.printNumInstances() # Also passes class to first argument
Number of instances: 2
class Spam:
numInstances = 0 # Trace class passed in
def __init__(self):
Spam.numInstances += 1
def printNumInstances(cls):
print("Number of instances:", cls.numInstances, cls)
printNumInstances = classmethod(printNumInstances)
class Sub(Spam):
def printNumInstances(cls): # Override a class method
print("Extra stuff...", cls) # But call back to original
Spam.printNumInstances()
printNumInstances = classmethod(printNumInstances)
class Other(Spam): pass # Inherit class method verbatim
>>> x, y = Sub(), Spam()
>>> x.printNumInstances() # Call from subclass instance
Extra stuff... <class 'test.Sub'>
Number of instances: 2 <class 'test.Spam'>
>>> Sub.printNumInstances() # Call from subclass itself
Extra stuff... <class 'test.Sub'>
Number of instances: 2 <class 'test.Spam'>
>>> y.printNumInstances()
Number of instances: 2 <class 'test.Spam'>
>>> z = Other()
>>> z.printNumInstances()
Number of instances: 3 <class 'test.Other'>
class Spam:
numInstances = 0
def count(cls): # Per-class instance counters
cls.numInstances += 1 # cls is lowest class above instance
def __init__(self):
self.count() # Passes self.__class__ to count
count = classmethod(count)
class Sub(Spam):
numInstances = 0
def __init__(self): # Redefines __init__
Spam.__init__(self)
class Other(Spam): # Inherits __init__
numInstances = 0
>>> x = Spam()
>>> y1, y2 = Sub(), Sub()
>>> z1, z2, z3 = Other(), Other(), Other()
>>> x.numInstances, y1.numInstances, z1.numInstances
(1, 2, 3)
>>> Spam.numInstances, Sub.numInstances, Other.numInstances
(1, 2, 3)
class C:
@staticmethod # Decoration syntax
def meth():
...
class C:
def meth():
...
meth = staticmethod(meth) # Rebind name
class Spam:
numInstances = 0
def __init__(self):
Spam.numInstances = Spam.numInstances + 1
@staticmethod
def printNumInstances():
print("Number of instances created: ", Spam.numInstances)
a = Spam()
b = Spam()
c = Spam()
Spam.printNumInstances() # Calls from both classes and instances work now!
a.printNumInstances() # Both print "Number of instances created: 3"
class tracer:
def __init__(self, func):
self.calls = 0
self.func = func
def __call__(self, *args):
self.calls += 1
print('call %s to %s' % (self.calls, self.func.__name__))
self.func(*args)
@tracer # Same as spam = tracer(spam)
def spam(a, b, c): # Wrap spam in a decorator object
print(a, b, c)
spam(1, 2, 3) # Really calls the tracer wrapper object
spam('a', 'b', 'c') # Invokes __call__ in class
spam(4, 5, 6) # __call__ adds logic and runs original object
def decorator(aClass): ...
@decorator
class C: ...
def decorator(aClass): ...
class C: ...
C = decorator(C)
def count(aClass):
aClass.numInstances = 0
return aClass # Return class itself, instead of a wrapper
@count
class Spam: ... # Same as Spam = count(Spam)
@count
class Sub(Spam): ... # numInstances = 0 not needed here
@count
class Other(Spam): ...
class Meta(type):
def __new__(meta, classname, supers, classdict): ...
class C(metaclass=Meta): ...
class C:
__metaclass__ = Meta
...
>>> class X:
... a = 1 # Class attribute
...
>>> I = X()
>>> I.a # Inherited by instance
1
>>> X.a
1
>>> X.a = 2 # May change more than X
>>> I.a # I changes too
2
>>> J = X() # J inherits from X's runtime values
>>> J.a # (but assigning to J.a changes a in J, not X or I)
2
class X: pass # Make a few attribute namespaces
class Y: pass
X.a = 1 # Use class attributes as variables
X.b = 2 # No instances anywhere to be found
X.c = 3
Y.a = X.a + X.b + X.c
for X.i in range(Y.a): print(X.i) # Prints 0..5
class Record: pass
X = Record()
X.name = 'bob'
X.job = 'Pizza maker'
>>> class C:
... shared = [] # Class attribute
... def __init__(self):
... self.perobj = [] # Instance attribute
...
>>> x = C() # Two instances
>>> y = C() # Implicitly share class attrs
>>> y.shared, y.perobj
([], [])
>>> x.shared.append('spam') # Impacts y's view too!
>>> x.perobj.append('spam') # Impacts x's data only
>>> x.shared, x.perobj
(['spam'], ['spam'])
>>> y.shared, y.perobj # y sees change made through x
(['spam'], [])
>>> C.shared # Stored on class and shared
['spam']
x.shared.append('spam') # Changes shared object attached to class in-place
x.shared = 'spam' # Changed or creates instance attribute attached to x
class ListTree:
def __str__(self): ...
class Super:
def __str__(self): ...
class Sub(ListTree, Super): # Get ListTree's __str__ by listing it first
x = Sub() # Inheritance searches ListTree before Super
class ListTree:
def __str__(self): ...
def other(self): ...
class Super:
def __str__(self): ...
def other(self): ...
class Sub(ListTree, Super): # Get ListTree's __str__ by listing it first
other = Super.other # But explicitly pick Super's version of other
def __init__(self):
...
x = Sub() # Inheritance searches Sub before ListTree/Super
class Sub(Super, ListTree): # Get Super's other by order
__str__ = Lister.__str__ # Explicitly pick Lister.__str__
def generate(): # Fails prior to Python 2.2, works later
class Spam:
count = 1
def method(self): # Name Spam not visible:
print(Spam.count) # not local (def), global (module), built-in
return Spam()
generate().method()
C:\python\examples> python nester.py
...error text omitted...
Print(Spam.count) # Not local (def), global (module), built-in
NameError: Spam
def generate():
global Spam # Force Spam to module scope
class Spam:
count = 1
def method(self):
print(Spam.count) # Works: in global (enclosing module)
return Spam()
generate().method() # Prints 1
def generate():
return Spam()
class Spam: # Define at top level of module
count = 1
def method(self):
print(Spam.count) # Works: in global (enclosing module)
generate().method()
def generate():
class Spam:
count = 1
def method(self):
print(self.__class__.count) # Works: qualify to get class
return Spam()
generate().method()
### lab code
class Lunch:
def __init__(self) # Make/embed Customer and Employee
def order(self, foodName) # Start a Customer order simulation
def result(self) # Ask the Customer what Food it has