python – Notes on the Python programming Language

• Python has an excellent language reference.
• Python has a style guide.

If you’re an experienced programmer new to Python then I recommend Python Tips by Muhammad Yasoob Ullah Khalid.

Implementations of Fundamental Types

List:

• e.g. [0,1,2]
• is a sequence
• is implemented as a variable-length array
  • NOT a linked list
• is a different data structure from array
  • See https://docs.python.org/2/library/array.html

Tuple:

• e.g. (0,1,2)
• is a sequence
• are immutable

Set:

• e.g. set([0,1,2])
• e.g. {0,1,2}
• is a sequence

Dictionary:

• e.g. { 'a':1 , 'b':2 }
• implemented with resizable hash tables
• any immutable object can be used as a dictionary key
• python exposes its hash function as the global hash() (32-bit)

Built-in functions

Use range(a) for loops. Note that range(a,b) returns an empty array if a and b are equal.

slice(start, stop) but use a[start:stop] instead

Numbers:

• int(x[, base]): constructor that can also be used to change base
• chr: character from integer as ASCII code
• ord: ASCII code to integer
• divmod(a,b): return (a // b, a % b) for integers
• bin, oct and hex
• abs
• pow: but use x**y instead

String:

• ord: return n

Iterables:

• len for length
• next(iterator[, default]), e.g. next(i, None)
• all and any: returns true if all/any element are true
• sorted
• reversed
• min and max
• map(function, iterable) and reduce(function, iterable[, initializer])
• filter(function, iterable) but use list comprehensions instead
• tuple to create a tuple from iterable
• zip([iterable, ...]): return list of tuples by “zippering” multiple lists
• enumerate: return list of numbered tuples
• Unfortunately, there is no some

imports

• from MODULE import NAME1, NAME2, ... to import NAME1 and NAME2 from MODULE
• from MODULE import * to import everything from MODULE
• from .MODULE is relative import (?)

Concurrency

• Python uses reference counting GIL: Global Interpreter Lock.
• There is only one lock!

TIPS

Functions can return multiple values:

def return_both(a,b):
  return a,b
x,y = return_both(1,2)

You can put multiple statements on one line by using a semi-colon, e.g. i += 1; j += 1 but this is discouraged. Perhaps only do this on a whiteboard or doc to save space.

Get an object’s reference count: sys.getrefcount(obj)

RELATED TOOLS

pip

pip is the python package manager

• sudo easy_install pip to install pip
• pip will show you available commands
  • pip search <keyword>
  • pip install <package>
  • pip freeze prints installed packages in “requirements” format
    • These are usually saved to requirements.txt
• pip help <command> will show you extended –help for a command

setuptools is the fundamental pip dependency

virtualenv

virtualenv creates a virtual environment for each program with:

• its own python
• its own pip

Installation:

• pip install virtualenv
• pip install virtualenvwrapper
• mkvirtualenv <name_of_env> to create a virtual env
• workon <name_of_env> to source (use) a virtual env

Virtual environments are not saved in you application folder. They are located in ~/.virtualenvs/name_of_env When you pip install a package that contains global binary then it is isntall in ~/.virtualenvs/name_of_env/bin

REFERENCES

A Byte of Python. Swaroop Chitlur.

A non-magical introduction to Pip and Virtualenv for Python beginners. Jamie Matthews. 2013-04-18.

Design and History FAQ. Python Documentation.

The Mighty Dictionary. Brandon Craig Rhodes. PyCon 2010. An optimized explanation of Python’s Dictionary implementation using a hash table.