Python

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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

Numbers

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])
str.split([char])

len(MyString) → returns the lenght of a string

Collections

List

Collection - allows us to put many values in a single variable. Simple variables are not collections. The previous value is overwritten when changed.

  • A List is made up of list 'constants'. Lists are surrounded by square brackets [] and the constants in the list are separated by commas. ([2,4,6,8])
  • A List element can be any Python object, even another list
  • A List can be empty
  • Lists are mutable (they can be changed)
  • When len() is used on a list, it counts the number of constants that make up the list. (not the number of characters)
  • Lists can be concatenated using +
  • Lists can be sliced
  • List is a unique type that can be checked using type() (result: <type 'list'>)
  • An empty list can be created with list()
  • Lists can be tested for contents using in/not in
  • List is an ordered sequence
  • A list can be sorted with .sort(). Sort changes the list permanently.
  • sorted(list) → Returns a list sorted.
  • Methods: append, count, extend, index, insert, pop, remove, reverse, sort
  • Functions len() - find length, max() - find highest value, min() - find lowest value, sum() - add all values average can be found with sum()/len()

Del Removes an item from a list.
example: a = [1, 2, 3, 4, 5] del a[0] returns [2, 3, 4, 5]

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

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)]

To sort by value: 

c = {'c':22, 'b':1, 'a':10}
tmp = list()
for k, v in c.items():
    tmp.append( (v, k) )
tmp.sort(reverse = True)
print tmp
[(22, 'c'), (10, 'a'), (1, 'b')]

The same sort by value with list comprehension 

c = {'c':22, 'b':1, 'a':10}
print sorted( [ (v,k) for k,v in c.items() ] )


Top 10 most common words. 

fhand = open('romeo.txt')
counts = dict()
for line in fhand:
    words = line.split()
    for word in words:
        counts[word] = counts.get(word, 0) +1
lst = list()
for key, val in counts.items():
    lst.append((val, key))
lst.sort(reverse = True)
for val, key in lst[:10]
    print key, val

Dictionary

Key - Value pairs. They are called diferent in diferent languages:
dictionary[key] = value

  • Perl → Associative arrays
  • Java → Properties, Map or HashMap
  • C# → Property bag


Declarationn and assigment:
There are two posible ways to declare them:

  • purse = dict()
  • puse = {}
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

Regular Expressions

^ → Matches the beginning of a line
$ →  Matches the end of a line
. → Matches any character
\s →  Matches any whitespace
\S →  Matches any non-whitespace
* → Repeats a character 0 or more times
*? →  Repeats a character 0 or more times (non-greedy)
+ → Repeats a character 1 or more times
+? → Repeats a character 1 or more times (non-greedy)
[aeiou] → Matches a single character in the listed set
[^XYZ] → Matches a single character NOT in the listed set
[a-z0-9] → The set of characters can include a range
( → Indicates where string extraction is to start
) → Indicates where string extraction is to end

\ → Escape character

Regular Expression Module

It must be imported at the begining of a program: 

import re
  • re.search(re_string, string) → similar to find()
  • re.findall(re_string, string) → similar to find, returns a list
  • re.match(re_string, string)
import re
hand = open('mbox-short.txt')
for line in hand:
    line = line.rstrip()
    if re.search('^From:', line);
        print line

Files

Class

MySQLdb

os.system()

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