Python


The basics



  • if / print
  • lists / dict / set
  • for / while
  • def
  • import
  • class

if

if 1 == 1:
    print "wow"
elif not "1" == 1:
    print "so much wow"
else:
    print "if all else fails trust the else"

The "Truth":

False True
False (== 0) True (== 1)
"" (empty string) any string but "" (" ", "anything")
0, 0.0 any number but 0 (1, 0.1, -1, 3.14)
[], (), {}, set() any non-empty container ([0], (None,), [''])
None almost any object that's not explicitly False

List []

myList = [1,2,"3"]
myList.append("test")
myList.remove("test") 
print myList[1]                                        # prints 2

Dict {}

myDict = {"key": "val"}
myDict['new key'] = "new val"
del myDict['new key']                                  # del is global
print myDict["key"]                                    # prints "val"

Set

mySet = set(1,2,3)
mySet.append(1)
mySet.remove(2)
print mySet[2]                                         # prints "3"

Common

len(collection)                                        # prints the size

for


myList = [1,2,3]
for var in myList:
    print var


while

while 1==1:
    print "Im crashing ur computer..."

ITERATING a dict

myDict = {"some key": "some val", 1: 2}

for key, val in myDict.items():
    print key, ":", val                 # prints some key : some val\n1 : 2

for key in myDict.keys():
    print key                           # prints some key\n1

for key in myDict:
    print key                           # prints some key\n1

DICTIONARIES AREN'T sorted

The dict 
myDict = {
    1:3,
    2:2,
    3:1
}
Simple 
for key, val in sorted(myDict.items()):
    print key, val
"Hard" 
for key, val in sorted(myDict.items(), key=lambda t: t[1]):
    print key, val
for key in sorted(myDict, key=lambda key: myDict[key]):
    print key, myDict[key]

Adding to dict tips


theList = [1,2,3,1,2,3,1,2,3,1,2,3,1,3,2,1,2,3,1,3,2,1] 
theDict = {}

The "wrong" way: 
for number in theList:
    if number not in theDict: 
        theDict[number] = 0
    theDict[number] += 1
The "right" way: 
for number in theList:
    theDict.setdefault(number, 0)
    theDict[number] += 1

Adding to dict tips


theList = [1,2,3,1,2,3,1,2,3,1,2,3,1,3,2,1,2,3,1,3,2,1] 
theDict = {}

The "wrong" way:
for number in theList:
    if number not in theDict: 
        theDict[number] = []
    theDict[number].append("BO!")
The "right" list way: 
for number in theList:
    theDict.setdefault(number, []).append("BO!")

Python 3.X vs 2.X


Simple explanation:
print "hello world 2.X"
print("hello world 3.X")

Print

print "one", "two"                           # prints "one two\n"
print("one", "two")                          # prints "one two\n"
print "no", "newline",                       # prints "no newline"
print("no", "newline", end="")               # prints "no newline"

FORMATTED print

print("vv%svv" % 123)                        # prints "vv123vv\n"
print("%s: %s" % ("one", "two"))             # prints "one: two\n"
print('%d-%0d-%5d-%05d' % (34, 34, 34, 34))  # prints "34-34-   34-00034\n"

Files


f = open('myfile.txt')                            # contains "1,2,3\n4,5,6"
for line in f:
    first, second, third = line.strip().split(',')
    print(second)                                 # prints 2\n5

Functions

def someFunc(someParam):
    return someParam + 1

Classes


class MyClass():
    member = 1

    def method(self, methodParam):
        return methodParam + self.member

Inheritance  
class MySecondClass(MyClass):
    def someThing(self):
        return member

Kill all getters/setters!


class myClass:
    x = "text"
Need to change behavior?
class C(object):
    _x = 0

    @property
    def x(self):
        return self._x

    @x.setter
    def x(self, value):
        self._x = value+2000

Class methods for OVERRIDING

  • object.__init__(self)
  • object.__del__(self)
  • object.__hash__(self)
  • object.__lt__(self, other)
  • object.__le__(self, other)
  • object.__eq__(self, other)
  • object.__ne__(self, other)
  • object.__gt__(self, other)
  • object.__add__(self, other)
  • object.__ge__(self, other)
  • object.__add__(self, other)

your first script


hello.py
 print("hello world")

Run it
 python hello.py

Using a main method


def main():
    print myfunk(1)

def myfunk(n):
    return n + 2

if __name__ == '__main__': main()

Importing


import filename

import filename, filename2

import method from filename

import method from filename as newName

standard libs 
sys <-- used to get argv

os  <-- used to get system variables and the like

re  <-- regex

The WEIRD stuff


":".join([1,2,3])                                 # becomes "1:2:3" 

elif:                                             # why not else if???

__someFunk__()                                    # "private" methods

Some of The good stuff

beautiful 
if 44 < var < 50:
    if var % 2:
        return true

auto unpacking

one, two, three = [1,2,3]
short for loops
oldArray = [[1,2],[3,4],[5,6]]
newArray = [x[1] for x in oldArray]
English
if not someTruthyVar: doSomething()

if someVar in list: doSomethingElse()
   

Exercises

  1. Create a script that takes an argument and prints it
  2. Count the lines in a file you send in as a argument
  3. Create a class and override the add method of the class
  4. Put the class in a separate file
  5. Create a function that takes unlimited
     arguments (not in these notes)
  6. Create a script that prints all the image urls in vg.no
    hint:
    from urllib import urlopen
    html = urlopen('http://vg.no').read()
  7. Create a script that prints the fibonacci series
  8. Create a script that finds the primes of a number

Python

By pylinux

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Python

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