python.md

Python Cheat Sheet

This is not an introduction to Python. For that, see the Python tutorial or the Python module index. Instead, this lists some tips and pointers that will prove useful when working on your language.

Lists

Lists will comprise our ASTs, so you'll need lists pretty early on. The tutorial page on lists should prove useful.

Dictionaries

We'll be using dictionaries when representing the program environments.

• Remember that dicts are mutable in Python. Use the copy function when a copy is needed.

• To update a dictionary with values from another, use update.

• You will find yourself needing to make a dictionary from a list of keys and a list of values. To do so, combine the dict and zip functions like this:

  >>> dict(zip(["foo", "bar"], [1, 2]))
  {'foo': 1, 'bar': 2}
  

Read more about dicts in the documentation.

Strings

• Strings works in many ways like lists. Thus, you can substring using indices:

  >>> "hello world"[6:]
  'world'
  

• Remove unwanted whitespace using str.strip().

• It is also useful to know about how to do string interpolation in Python.

  >>> "Hey, %s language!" % "cool"
  'Hey, cool language!'
  >>> "%d bottles of %s on the wall" % (99, "beer")
  '99 bottles of beer on the wall'
  >>> "%(num)s bottles of %(what)s on the %(where)s, %(num)d bottles of %(what)s" \
  ...      % {"num": 99, "what": "beer", "where": "wall"}
  '99 bottles of beer on the wall, 99 bottles of beer'
  

Classes

When defining a class, all methods take a special argument self as the first argument. The __init__ method works as constructor for the class.

class Knight:
    
    def __init__(self, sound):
        self.sound = sound
    
    def speak(self):
        print self.sound

You don't provide the self when creating instances or calling methods:

>>> knight = Knight("ni")
>>> knight.speak()
ni

Default argument values

One thing it is easy to be bitten by is the way Python handles default function argument values. These are members of the function itself, and not "reset" every time the function is called.

>>> def function(data=[]):
...     data.append(1)
...     return data
...
>>> function()
[1]
>>> function()
[1, 1]
>>> function()
[1, 1, 1]

Beware this when you implement the Environment class.