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7.5 Decorated Methods
This section discusses a few built-in decorators that are used in combination with method definitions.
Predefined Decorators
There are predefined decorators used to specify special kinds of methods in class definitions.
class Foo:
def bar(self,a):
...
@staticmethod
def spam(a):
...
@classmethod
def grok(cls,a):
...
@property
def name(self):
...
Let's go one by one.
Static Methods
@staticmethod
is used to define a so-called static class methods
(from C++/Java). A static method is a function that is part of the
class, but which does not operate on instances.
class Foo(object):
@staticmethod
def bar(x):
print('x =', x)
>>> Foo.bar(2) x=2
>>>
Static methods are sometimes used to implement internal supporting code for a class. For example, code to help manage created instances (memory management, system resources, persistence, locking, etc). They're also used by certain design patterns (not discussed here).
Class Methods
@classmethod
is used to define class methods. A class method is a
method that receives the class object as the first parameter instead
of the instance.
class Foo:
def bar(self):
print(self)
@classmethod
def spam(cls):
print(cls)
>>> f = Foo()
>>> f.bar()
<__main__.Foo object at 0x971690> # The instance `f`
>>> Foo.spam()
<class '__main__.Foo'> # The class `Foo`
>>>
Class methods are most often used as a tool for defining alternate constructors.
class Date:
def __init__(self,year,month,day):
self.year = year
self.month = month
self.day = day
@classmethod
def today(cls):
# Notice how the class is passed as an argument
tm = time.localtime()
# And used to create a new instance
return cls(tm.tm_year, tm.tm_mon, tm.tm_mday)
d = Date.today()
Class methods solve some tricky problems with features like inheritance.
class Date:
...
@classmethod
def today(cls):
# Gets the correct class (e.g. `NewDate`)
tm = time.localtime()
return cls(tm.tm_year, tm.tm_mon, tm.tm_mday)
class NewDate(Date):
...
d = NewDate.today()
Exercises
Exercise 7.11: Class Methods in Practice
In your report.py
and portfolio.py
files, the creation of a Portfolio
object is a bit muddled. For example, the report.py
program has code like this:
def read_portfolio(filename, **opts):
'''
Read a stock portfolio file into a list of dictionaries with keys
name, shares, and price.
'''
with open(filename) as lines:
portdicts = fileparse.parse_csv(lines,
select=['name','shares','price'],
types=[str,int,float],
**opts)
portfolio = [ Stock(**d) for d in portdicts ]
return Portfolio(portfolio)
and the portfolio.py
file defines Portfolio()
with an odd initializer
like this:
class Portfolio:
def __init__(self, holdings):
self.holdings = holdings
...
Frankly, the chain of responsibility is all a bit confusing because the
code is scattered. If a Portfolio
class is supposed to contain
a list of Stock
instances, maybe you should change the class to be a bit more clear.
Like this:
# portfolio.py
import stock
class Portfolio:
def __init__(self):
self.holdings = []
def append(self, holding):
if not isinstance(holding, stock.Stock):
raise TypeError('Expected a Stock instance')
self.holdings.append(holding)
...
If you want to read a portfolio from a CSV file, maybe you should make a class method for it:
# portfolio.py
import fileparse
import stock
class Portfolio:
def __init__(self):
self.holdings = []
def append(self, holding):
if not isinstance(holding, stock.Stock):
raise TypeError('Expected a Stock instance')
self.holdings.append(holding)
@classmethod
def from_csv(cls, lines, **opts):
self = cls()
portdicts = fileparse.parse_csv(lines,
select=['name','shares','price'],
types=[str,int,float],
**opts)
for d in portdicts:
self.append(stock.Stock(**d))
return self
To use this new Portfolio class, you can now write code like this:
>>> from portfolio import Portfolio
>>> with open('Data/portfolio.csv') as lines:
... port = Portfolio.from_csv(lines)
...
>>>
Make these changes to the Portfolio
class and modify the report.py
code to use the class method.
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