Python

Intro

Python 2 and 3 differences

print "fred" // OK Python 2
print("fred") // Not OK Python 2

Whitespace

Uses full colon and four spaces instead of brackets e.g.

for i in range(5):
    x = i * 10
    print(x)

Rules

• Prefer four spaces
• Never mix spaces and tabs
• Be consistent on consecutive lines
• Only deviate to improve readability

Help

help(object) gives help. e.g. for the module math

help(math)

Scalar Types, Operators and Control

Types

• int (42)
• float (4.2)
• NoneType (None)
• bool ( True, False) 0 = False !=0 = True

Operators

• == value equality
• != value inequality
• < less-than
• > greater-than
• <= less-than or equal
• >= greater-than or equal

Control

if statementes

if True: 
    print("Its true")

h = 42
if h > 50: 
    print("Greater than 50")
elif h < 20:
    print("Less than 20")
else:
    print("Other")

while loops

while c != 0: 
    print(c)
    c -= 1 // c = c-1
    print("Its true")

while True: 
    response = input()
    if int(response) % 7 == 0: 
       break

for loops

cities = ["London", "Paris", "Berlin"]

for city in cities:
  print(city)

Collections and Iterations

str

Double and single quotes are supported. Strings are immutable. Multiline

"""This is 
a multiline
string"""

m = "This string\nspans multiple\nlines"

Raw Strings like c# @

path = r'C:\users\merlin\Documents'

bytes

These work like strings, well ascii strings as and can be created like below

b'some bytpes'
print(b[0]) // 115

decoding to bytes

norsk = "some norsk characters"
data = norsk.encode('utf8')
norwegian = data.decode('utf8')

lists

List are a sequence of lists

m = [1,14,5]

// Can be different types
m = ['apple', 7, false]

// Add are mutable
b = []
b.append(1.666)
b.append(1.4444)
print(b) // [1.666, 1.4444]

// Constructor
print(list("characters")) // ['c','h','a','r','a','c','t','e','r','s']

Dict

Dict are value pairs

m = {'1': 'Apple', '2': 'Orange'}

print(m['1']) // Apple

// Replaces
m['1'] = 'Banana']
print(m['1']) // Banana

Modularity

Functions

General

These are created as below

def foo(arg1, arg2):
    return arg1 * arg2

Defaults

def foo(arg1, arg2=9):
    return arg1 * arg2

Be aware that the def assignment is only run once. Therefore These are created as below

def add_spam(menu=[]):
    menu.append('spam')

add_spam() // ['spam']
add_spam() // ['spam','spam']

Advice is to make default arguments not mutable. i.e. not strings and not ints

def add_spam(menu=None):
    if(menu==None)
       menu = []
    menu.append('spam')
    return menu

add_spam() // ['spam']
add_spam() // ['spam']

Importing defs

Best to be selective

from words import (fetch_words, print_words)

// could be BAD BAD!!
from words import *

Passing arguments

import sys

if __name__ == '__main__':
    main(sys.argv[1])

Comments

def fetch_words(url):
    """Fetch a list of words from a URL.
    
    Args:
        url: The URL of UTF-8 text document.

    Return:
        A list of strings containing the words from
         the document.
    """
    story = urlopen(url)
    story_words = []
    for line in story:
        line_words = line.decode('utf8').split()
        for word in line_words:
            story_words.append(word)
    story.close()
    return story_words

Scope of Objects

• Local - Inside current function
• Enclosing - Inside enclosing function
• Global - At the top level of the module
• Built-in - In the special builtins module

Objects and Types

Named references to objects

Assigning variables is the same as references. Use id() to prove this.

s = [1,2,3]
r = s
s[0] = 500
print(r)
[500,2,3]

p = [4,5,6]
q = [4,5,6]
print(p == q) // True
print(p is q) // False

Passing Arguments are like references

Passing arguments is like passing references

m = [9,15,24]
def modify(k):
    k.append(39)
    print("k = ", k)

modify(m)
k = [9,15,24, 39]
print(m)
[9,15,24, 39]

Passing Arguments are like references II

Or are they. g is reassigned not mutated

f = [14, 23, 37]
def replace(g):
    g = [17,28, 45]
    print("g = ", g)

replace(f)
g = [17,28, 45]
print(f)
[14,23,37]