Python (L21-L25)

// - integer division

% - Modulo

** - exponentiation

Multiple assignment:

• x, y, z = 1, 2, 3

x += 2 is the same as x = x + 2

Same applies to *= and -=

All divisions will return a float. Can use type() to see type of variable.

Convert with int() and float()

Strings

Backslash escapes strings

+ to concat

* to repeat

Data structures

lists: [1, 2, 3]

slicing a string or list: months[6:9] (from and including, to but not including)

in, not in -> membership operators

"This" in "This is a test" -> true

Tuples: Immutable, ordered: location = (12.3516, 35.2346). Parenthesis can be omitted

Tuple unpacking:

• dimensions = 52, 40, 100
• length, width, height = dimensions

Sets - Collecting unique objects

• unique elements without ordering
• set([foo, bar])
• union etc operators can be used and are much faster than with other containers

Dictionary

• Mutable oebject that store mappings of unique keys to values. Key value store
• elements = {'hydrogen': 1}, print elements['hydrogen']

Compound data structures

• Dictionaries in dictionaries: animals = {'fish': {'shark': 1}}, animals['fish']['shark'] # 1

Syntax: if, elif, else

Here are most of the built-in objects that are considered False in Python:

• constants defined to be false: None and False
• zero of any numeric type: 0, 0.0, 0j, Decimal(0), Fraction(0, 1)
• empty sequences and collections: '"", (), [], {}, set(), range(0)

FOR-loops: for city in cities:

range(start, stop, step)

• range(4) = [0, 1, 2, 3] (stop-1)
• range(2, 6) = [2, 3, 4, 5] (start, stop-1)
• range(1, 10, 2) = [1, 3, 5, 7, 9] (start, stop-1, step)
• for index in range(len(cities)):
  • cities[index] = cities[index].title()

WHILE-loops: while condition:

BREAK to terminate a loop

CONTINUE to go to the next iteration

zip returns an iterator that combines multiple iterables into one sequence of tubles

enumerate adds indexes

list comprehension is shorthand syntax for creating lists. Can also use if/else to filter.

Define function with def function_name(arguments):

Call functions with arguments either by argument position or name

Docstrings allows to add documentation to for instance function

Lambda expressions: Anonymous functions

Iterator: Object that represents a stream of data.

Generator: Function that creates an iterator, has yield

import filename (same folder) creates an object "filename" of type "module"

import named "filename" is referred to as both a script and a module

a module is just a file with python definitions and statements

variables can be accessed with filename.variable

make an alias for imports: import some_long_filename as alias

The if main-block: use if __name__ == '__main__': to only execute code when the current file is being executed, not when it is imported. Good practice to write executable statements inside an "if name"-block. Alternatively define a main() function and run this in the "if main"-block.

Python Standard Library - ready-made modules, see docs and example code.

Other variants of imports

• from collections import defaultdict: Only import parts of a module. Don't have to use dot notation to access the imported libs.
• from collections import defaultdict, namedtuples: Import multiple parts of a module with comma.
• With aliasing: from csv import reader as csvreader
• Antipattern: from random import * (may cause naming collisions, makes it impossible to find where code was imported)

A package is a module that contains submodules. Example: os.path. Usage: os.path.isdir('my_path')

Import submodules with import package_name.submodule_name

Third party libraries

Install libraries using pip: pip install pytz (a timezone package)

• Convention is to import third party libraries after standard libraries
• package requirements are often listed in requirements.txt, with version numbers
• install all dependencies: pip install -r requirements.txt