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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.