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Common Python stuff with syntax.

Python: Remember uses American English, not normal English

Importing Libraries

import thing to import here

Eg:

import math

Input

Input with type interpretation:

variable = input("Prompt for input")

Input to string:

variable = raw_input("Prompt for input")

Type conversion

To integer:

int("string")

To string:

str(12)

String to list (each letter):

list(string)

String to list (each word):

string.split(' ')

List to string:

''.join(list)

where '' contains the separator.

Strings & lists

Many operations on the two types are the same. If our example string is called "string", and our example list is called "list":

Concatenation

uses + for both types, but must be same type being concatenated.

Length

len(string)

or

len(list)

len(something) will return value equal to the last index + 1

Letters in string

Letter n in string "string" is represented by:

"string"[n-1]

Last letter in string:

"string"[-1]

Show letters from letter 3 to 7 (indexes 2 to 6)

"string"[2:7]

Letters up to & including letter 8 (indexes to 7)

"string"[:8]

Letters after letter 2 (indexes from 1)

"string"[2:]

Reverse string order

string[::-1]

Change case

string.lower()
string.upper()

Replace

In string:

string.replace(oldSubString, newSubString)

Append item to list

list.append(newItem)

Sort

list.sort()

Strip whitespace

Both sides:

string.strip()

Just from left:

string.lstrip()

Just from right:

string.rstrip()

Find in string

String at the beginning:

string.startswith(subString)

String at end:

string.endswith(subString)

Formatting strings

In general, shown as the following:

print "%d is %s" % (123, "good")

Kinds of formatting:

%s -- string
%d -- display number
%10d -- display number within field of 10 characters (rightmost side)
%f -- floating point numbers (default field 8 digits long)
%5.3f -- float in field of 5 with precision 3
%-5.3f -- same as above, but left adjusted
%+5.3f -- same as two above, but includes sign

Values taken from tuple in same order.

Dictionaries

Enclosed in curly braces as follows:

 myDictionary = {'dog':1, 'cat':0, 'sister':'annoying little twerp'}

Call with key values:

myDictionary['dog'] 

will return

1

Keys

List of keys:

dictionary.keys()

Number of keys:

len(dictionary.keys())

Other

Check for existence of key (returns Boolean):

dictionary.has_key(key)

Remove and return item:

dictionary.pop(key)

Tuples

Immutable lists displayed with round rather than square brackets.

Boolean

Comparisons

==

equality

!=

inequality

>  >=

Greter than/ greater than or equals to

< <=

Less than/ less than or equals to

Range:

range(0,10,2)

counts from 0 to ten (not inclusive of 10) in steps of 2.

And & or are litterally written as

and

&

or

Files

Open a file:

f = open('file.extension', 'rU')

Read next line:

f.readline()

Read all lines (list with one item per line):

f.readlines()

Note: will probably contain trailing \n.

Close file:

f.close()

Functions

Define a function:

def hello(name):
    Stuff goes here

Call a function:

hello(args)

A function with named parameters with default values:

def foo(x=1, y=9, z=6):
    print x, y, z
foo(z=1,x=2)
# => 2 9 1

Can use an asterisk to parse in arguments as a list.

Can use two asterisks to parse in arguments as a dictionary:

def bar(**params):
    print params
bar('p'=1, 'q'=2)

returns {'p':1, 'q':2}

Handy for loops

Access one line of a file at a time:

for line in f.readlines():
    Stuff goes here

Cycle through the key/value pairs in a dictionary:

for key, value in dictionary.iteritems():
    Stuff goes here

(Also iterkeys and itervalues to iterate over only one or the other)

Testing

import doctest

def function(variables):
    """This is some documentaion
    
    Here are some test cases:
    >>> function(1)
    expectedResult

    >>> function(2)
    anotherExpectedResult
    """
    
doctest.testmod()

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