42.3.1. Indentation

if a:
    statement1
    statement2    # Consistent indentation
else:
    statement3
      statement4  # Inconsistent indentation (error)

Python assumes eight-space tab characters. If you have your editor set to four-space tabs, for example, this can bite you if there are mixed spaces and tabs. Either use eight-space tabs, or stick to spaces.

>>> a = math.cos( 3 * ( x - n ) ) + \
...     math.sin( 3 * ( y - n ) )

Lines that are already grouped within triple-quotes, parentheses (...), brackets [...], or braces {...} can span multiple lines without needing to use the backslash.

42.3.2. Functions

>>> def add(a, b):
...     return a + b

This defines a function and attaches it to the name add in the current namespace; anything with access to this namespace can call this function by simply passing arguments to it:

>>> print add(3, 5)
8

>>> add = lambda a, b: a + b

>>> def timesThree(a):
...     return 3 * a
>>> def sum(x, y):
...     return x + y

>>> ints = [1, 2, 3, 4, 5]
>>> multiples = map(timesThree, ints)
>>> print multiples
[3, 6, 9, 12, 15]
>>> print reduce(sum, multiples)
45

>>> ints = [1, 2, 3, 4, 5]
>>> multiples = map(lambda a: 3 * a, ints)
>>> print multiples
[3, 6, 9, 12, 15]
>>> print reduce(lambda x, y: x + y, multiples)
45

42.3.3. Everything's an Object

class Account:
    "A simple example class"
    kind = "Checking"
    def _ _init_ _(self, accountHolder, startingBalance):
        self.accountHolder = accountHolder;
        self.balance = startingBalance;

>>> account = Account("Deb", 86753.09)

This creates a new Account object and sets the accountHolder instance variable to Deb and the balance instance variable to $86,753.09. Now, in order to be able to do anything with our Account, we need to define methods to allow manipulation of the balance:

class Account:
    ...
    def deposit(self, depositAmount):
        "Deposit money"
        self.balance = self.balance + depositAmount
    def withdraw(self, withdrawalAmount):
        "Withdraw money"
        self.balance = self.balance - withdrawalAmount
    def inquireBalance(self):
        "Balance inquiry"
        return self.balance

>>> account.deposit(1504.36)
>>> account.withdraw(40.00)
>>> print "Account balance is now $%.2f" % account.inquireBalance( )
Account balance is now $88217.45

42.3.4. Modules and Packages

import account

checking = account.Account("Deb", 86753.09)

Note that we can't refer to Account directly; we have to refer to it through its imported name, account.Account. If, for convenience, we'd like to access the Account class directly, we can tell Python to import the class into our current namespace as well:

from account import Account

checking = Account("Deb", 86753.09)

Modules are compiled into bytecodes the first time they are imported, allowing them to run faster and be more compact.

Given that a Python module is just a file, it will probably come as no surprise that a Python package is simply a directory with modules in it. To tag a directory as a package rather than just any directory, create a file called _ _init_ _.py (the same name as the method to initialize an object) within that directory. Code within _ _init_ _.py will get run whenever any part of its package is imported. Subpackages are, of course, just subdirectories with their own _ _init_ _.py files.

42.3.5. I/O and Formatting

>>> print "Account balance is now $%.2f" % account.inquireBalance( )
Account balance is now $86753.09
>>> print "Error:  %s(%s)." % (error, error.number)
Error:  File not found(2)

% takes a string and a list of arguments. (If there's only one argument, it can be any object instead of a list.) Any place that you might want to use a string, you can use the string formatting operator. For example:

>>> obj.name = "MyObject: %s" % name
>>> url = urlopen("%s://%s:%d/%s" % (protocol, host, port, path))

42.3.6. wxPython