Python

Indentation

Python is an indented language, so the code indentation matters. A good practice is to indent with 4 spaces (if you mix spaces and tabs the code won't work.

Operators

Arithmetic

+    Addition
-    Substraction
*    Multiplication
/    Division
**   Power
%    Reminder
//   Floor division (Reminder is removed)

Comparison

==   Equal 
!=   Not equal
>    Greater
>=   Greater or equal
<    Smaller
<=   Smaller or equal
<>   Similar to !=

Assignment

=    Simple assignment
+=   Add and as
-=   Substract and  assignment
*=   Multiply and assignment
/=   Divide and assignment
%=   Modulus and assignment
**=  Exponent and assignment
//=  Floor Divisionn and assignment

Bitwise operators

They perform operations on binary terms. a= 8 → 100; b= 9 → 101; a & b → 100

$    Binary AND
|    Binary OR
^    Binary XOR
~    Binary complement
<<   Binary left shift
>>   Binary right shift

Logic

and
or
not

Membership operators

in
not in

Variables

String

String assingment:
MyString = 'Hello World' OR MyString = "Hello World"
Strings can be subset sliced:

MyString = "Hello world"
MyString[0:4]

Hel

in operator: to check if a substring is contained in a string

String library:

str.lower()
str.upper()
str.capitalize() → Uppercases 1st char
str.center(width[, fillchar])
str.startswith(preffix[, start[, end]])
str.endswith(suffix[, start[, end]])
str.find(substring[, start[, end]])
str.lstrip([chars])
str.rstrip([chars])
str.strip([chars])
str.replace(old, new[, count])

len(MyString) → returns the lenght of a string

Collections

List

• sorted(list) → Returns a list sorted.

Tuple

Inmutable, declared by ()

• My_tuple = (3, 2, 1)

You can iterate trough tuples.
You can also refer to the items of a tuple like in a list.

My_tuple[0]
3

They support multple assignment, usefull for example to iterate trough dictionaries:

d = dict()
d['steve'] = 2
d['susan'] = 4

for (k, v) in d.items():
    print k, v

Tuples are comparable. They compare the leftmost first, then the next and so on...

>>> (0, 1, 2) < (5, 1, 2)
True
>>> (0, 1, 30000000) < (0, 3, 1)
True

This can be used to sort dictionaries by keys.

>>> d = {'c':22, 'b':1, 'a':10}
>>> t = d.items()
>>> t
[('c', 22), ('b',1), ('a', 10)]
>>> t.sort()
>>> t
[('a', 10), ('b',1), ('c', 22)]

==== Dictionary ====
Key - Value pairs. They are called diferent in diferent languages:<br />
dictionary[key] = value <br />

* Perl → Associative arrays
* Java → Properties, Map or HashMap
* C# →  Property bag
<br />
Declarationn and assigment:<br />
There are two posible ways to declare them:
* purse = dict()
* puse = {}
<source lang="python">
purse = dict()
purse['money'] = 12
purse['candy'] = 3

print purse['money']

purse = {'money': 12, 'candy' = 3}

get() method:
If you try to retrieve a value for a key that doesn't exist you would get a traceback error.
To avoid this you should use the get() method, that returns the default value if the key doesn't exist: purse.get(name, default_value) Other methods and functions

• list(purse) → Returns a list of keys.
• dict.keys() → Returns a list of keys.
• dict.values() → Returns a list of values.
• dict.items → Returns a list of tuples ( [(key, value), (key, value)...] )

Word count using files and dictionarys

counts = dict()
file_path = raw_input('Enter file name: ')
file_handle = open(file_path, 'r')
text = file_handle.read()
words = text.split()

for word in words:
    counts[word] = counts.get(word, 0) + 1

bigCount = None
bigWord  = None
for word, count in counts.items():
    if bigCount is None or count > bigCount:
        bigWord = word
        bigCount = count

print 'Most frequent word: ', bigWord, '\nFrequency: ', bigCount'

Conditional

if/elif/else

if a < 10:
    print "Less than 10"
elif a >= 10 and a < 20:
    print "a greater or equal to 10 and less than 20"
else:
    print "a greater or equal to 20"

try/except/finally

try:
    file = open("test.txt")
except:
    print "Could not open file"
finally:
    print "This part will be executed at the end whether the open fails or not"

Loops

For

NumberList = [1, 3, 7, 12, 24]
for number in NumberList:
    print number

Another way:

for i in range(0, len(NumberList)-1):
    print NumberList[i]

while

CtrlNum = 7
while CtrllNum > 3:
    print CtrlNum
    CtrlNum -= 1

Functions

All arguments in Python are passed by reference, if you change a variable value inside a function it will be changed at the calling function.

def MyFunction(formal_arg, optional_arg = None, *variable_lenght_args):
    print formal_arg
    if optional_arg:
        print optional_arg
    for arg in variable_lenght_args:
        print arg