Python

Variables

Just use them, no scoping.

a = 3

Types

• int
• str
  • .upper(), .lower(), .capitalize(), .title()
  • .isdigit(), isalpha()
  • .strip(), .lstrip(), .rstrip() - Can be used for specific characters
  • .count("word"), .find("word"), .replace("old", "new")
  • /n and /t
  • Interpolate with "First: {first}, Second: {second}".format(first="first", second="second")
• float
• bool
  • True / False

Collections

• dict - Key:Value
  • {a: 1, b: 2}
  • .pop(key) - Removes
  • .has_key(key)
  • .keys(), .values()
  • ==
• list - A general collection
  • [1, 2, 3,]
  • list[0]
  • del list
  • list[1:3] - Second through 3rd elements
  • .append(item), .extend(list), .remove(firstMatch), .insert(position, element)
  • .sort(), .reverse()
  • ==, !=, + (concat), * (repeat)
• tuple - An immutable list
  • (1, 2, 3,)
• set - Unique, unordered
  • {1, 2, 3}

Input

• input("prompt") - Get input from the console, try to match data type
• raw_input("prompt") - Get input from the console, as a string
• open() - Get a file pointer
  • .readlines()
  • .close() when done
• json.load(file_pointer)
• getpass("prompt") - Get hidden input, must be imported with from getpass import getpass

Output

• print() - Output to the console
• format() - Apply variables to a string
• open() - Get a file pointer
  • .writelines(content)
  • .write(content) - One line
  • Open a file for writing with open("filename", "w")
  • Open a file for appending with open("filename", "a")
  • Create a file for writing with open("filename", "w+")
  • .close() when done
• json.dump(dictionary, file_pointer) - Print to file
• json.dumps(dictionary, indent=4) - Print to screen

Loops

• range(start, end, increment)
• continue, break

For

for element in list:
    #

While

age = raw_input("Age?")
while not age.isdigit():
    print "Oops"
    age = raw_input("No, your age:")

Functions

• Arguments must match number of paramenters
  • If variable args, use **kwargs (key:value) or *args (tuple)
• Optional parameters have to come last
• Can return multiple, values (will come back as a tuple)
• Can destructure, returns
• Simply pass by value/reference rules as JS

def name(parameter1, parameter2=''):
    #
    return

Conditionals

if some_condition:
    #
elif other_condition:
    #
else:
    #

• Each type has their own falsy value
• not is the negation operator

Errors

try:
    #
except:
    #

Math

• +/-/*///%/**
• abs()
• Negate with -
• If the result of a division should be a float, one of the numbers has to be a float (either literal or with float()

Built-Ins

• type()
• len() - Arrays or Strings

Classes & Objects

class ClassName(ParentClass):
    def __init__(self, parameter, option=""):
        super(ClassName, self).__init__()
        self.parameter = parameter
    def some_method(self):
        return self.parameter
    def __eq__(self, other): // Allows == comparison
        return self.parameter == other.parameter
    // Also can do eq, ne, gt, lt, gte, lte, str

class_name = ClassName()

Modules

• Include a file called __init__.py in any folder you want to add to the path
• Use underscores instead of dashes for names
• The main() function is the conventional point of entry for the app

import module
from module import class_or_function
from directory.file_name import class_or_function
from module import *

Popular modules:

• random - .randint(), .random(), .uniform(). .choice()
• os
• json
• sqlite3
• datetime
• getpass
• pprint

Resolution Order

1. Standard library
2. Current directory
3. External libraries

Comments

• #
• Triple ticks define docstrings, which will show up when passed into help()

OS

• os.getcwd() - Current runpath
• os.listdir('/etc') - List files in current directory or passed in dir
• os.makedir('name') / os.makedirs('name/nested')
• os.stat() - Get file info

Debugging

Steps through a file line by line:

python -n pdb filename.py

Commands

• a - What arguments are being passed to this function
• b - Adds a breakpoint
• c - Continues to next break point
• cl - Clear all breakpoints
• l - List the code around your position
• n - Execute the next line of code
• s - Step through this line, stopping if you enter a function
• variable_name - Current value of variable