Python Cheat Sheet

I've been playing around with python over the last few days. It's really easy to use and I've written the following cheat sheet so that I don't forget how to use it:

Collections:

###################### TUPLE - immutable list
rgb = ("red", "green", "blue")

assert len(rgb) == 3
assert min(rgb) == "blue"
assert max(rgb) == "red"

#find the last element
assert rgb[len(rgb) - 1] == "blue"

assert "red" in rgb
assert "yellow" not in rgb

#slice the list [1,3)
rgb[1:3] #('green', 'blue')

#concatenate twice
rgb * 2

#iterate over list
for e in rgb:
 print e

###################### LIST - mutable list
colors = ["black", "white"]
assert len(colors) == 2

#add an element
colors.append("green")
assert len(colors) == 3
assert colors[2] == "green"

#remove an element
colors.remove("white")
assert "white" not in colors

colors.append("green")

#count occurrences of an element
assert colors.count("green") == 2

#return first index of value
assert colors.index("black") == 0

#insert element
colors.insert(1, "orange")
assert colors.index("orange") == 1

#remove the last element
e = colors.pop()
assert e == "green"

#replace a slice
colors[1:3] = ["red"];

colors.reverse()
colors.sort()

###################### SET
l = ["a", "b", "a", "c"]
assert len(set(l)) == 3

###################### DICTIONARY- mappings

dict = {"alice":22, "bob":20}
assert dict.get("alice") == 22

#add an element
dict["charlie"] = 25
assert dict.get("charlie") == 25

#get list of keys
assert len(dict.keys()) == 3

#get list of values
assert len(dict.values()) == 3

#check for key
assert dict.has_key("alice")

Loops:

for i in range(1, 10, 2):
    print i

while True:
    print 1
    break

Methods:
Printing out the factorial of a number.

def factorial(n):
    """prints factorial of n using recursion."""
    if n == 0:
        return 1
    else:
        return n * factorial(n - 1)

print factorial(4)

Classes:

class Person:

    def __init__(self, name, age):
        self.name = name
        self.age = age

    def equals(self, p):
        return isinstance(p, Person) and \
               self.name == p.name and \
               self.age == p.age


    def toString(self):
        return "name: %s, age: %s" % (self.name, self.age)


p = Person("Bob", 20)
q = Person("Charlie", 20)
assert p.equals(p)
assert p.equals(q)==False
print p.toString()

Read user input:

string = raw_input("Enter your name: ")
number = input("Enter a number:")

File I/O:

#write to file
filename = "C:/temp/test.txt"
f = file(filename, "w")
f.write("HELLO WORLD\nHELLO WORLD")
f.close()

#open a file in read-mode
f = file(filename, "r")

#read all lines into a list in one go
assert len(f.readlines()) == 2

#print out current position
print f.tell()

#go to beginning of file
f.seek(0)

#read file line by line with auto-cleanup
with open("C:\\temp\hello.wsdl") as f:
    for line in f:
        print line

Pickling (Serialisation):

#pickling
f = file("C:\\temp\dictionary.txt", 'w')
dict = {"alice":22, "bob":20}
import pickle
pickle.dump(dict, f)
f.close()

#unpickling
f = file("C:\\temp\dictionary.txt", 'r')
obj = pickle.load(f)
f.close()
assert obj["alice"] == 22
assert obj["bob"] == 20

Exception handling:

try:
    i = int("Hello")
except ValueError:
    i = -1
finally:
    print "finally"


#creating a custom exception class
from repr import repr

class MyException(Exception):
    def __init__(self, value):
        self.value = value
    def __str__(self):
        return repr(self.value)

try:
    raise MyException(2*2)
except MyException as e:
    print 'My exception occurred, value:', e.value

Lambdas:

def make_incrementor (n):
    return lambda x: x + n

f = make_incrementor(2)
print f(42)

print map(lambda x: x % 2, range(10))