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Languages like Java and C++ have lots of rules regarding variable types, syntax, return values, and so on.  Although these restrictions help make the compiled program run quickly, they are cumbersome when you are trying to write short, quick scripts to perform tasks.  This is where a ''scripting language'' comes into play.

'''Python''' is a language well-suited to rapid prototype development.  It is an ''interpreted language'', which means that you do '''''not''''' need to compile the code when you run it.  The syntax is clean, and it is usually clear at first glance what is going on when you write in Python.

{{XKCD
|name=python
|id=353
}}

== Installation ==

Python may already be installed on your system.  To see whether or not it is, enter the command

<source lang="bash">$ python --version</source>

If it tells you a version of Python (like "2.7.1"), then you're good to go.  If not, you need to do a quick package install to get it up and running.  Apt and Yum both call a functional Python package '''python'''.

=== Pip ===

Linux distributions have package managers like ''Apt'', ''Yum'', and ''YaST''.  PHP has a package manager named ''PEAR''.  It's now time to introduce Python's leading package manager: '''pip'''.

You need to install Pip from Apt or Yum before you can use it.  Both call the package '''python-pip'''.

Once you have pip installed, you can use it to install Python packages.  Use the '''pip-python''' (RHEL) or '''pip''' (Debian) command:

<source lang="bash">
$ pip-python install package_name # RHEL
$ pip install package_name # Debian
</source>

== Running Python ==

There are two common ways to run Python code: via the console, and via a Python script file.

=== The Python Console ===

The Python console enables you to experiment with code without opening a text editor.  To enter the Python console, simply type the ''python'' command at the terminal:

<source lang="bash">$ python</source>

To leave the interactive console, either type "quit()" or press Ctrl-D (on both Mac and Windows).

=== Python Script Files ===

You can also save Python script files for later use.  The extension for Python scripts is '''*.py'''.  To run a script file, simply feed its path as an argument to the ''python'' command in Terminal:

<source lang="bash">$ python my_script.py</source>

== Python Syntax and Language Components ==

This section contains a very brief overview of Python syntax.  For a more comprehensive introduction, see [http://docs.python.org/tutorial/ the Python docs].

=== An Example Python Script ===

In my mind, there's no better way to learn Python than to be immersed in a simple example script.

<source lang="python">
print "Hello World"

fruits = ["apple", "banana", "cherry", "date"]
for fruit in fruits:
	print "I always love to eat a fresh %s." % fruit

# Map the fruits list over to a new list containing the length of the fruit strings:
fruit_size = [len(fruit) for fruit in fruits]

avg_fruit_size = sum(fruit_size) / float(len(fruit_size))
print "The average fruit string length is %4.2f." % avg_fruit_size
</source>

Some things to notice:
* Printing is achieved using the '''print''' command
* A colon starts a block, similar to a curly brace '''{''' in many other languages.  The corresponding code block '''''must''''' be indented.  The end of the code block is signified by when the indentation ends.
* Strings can be printf-style formatted using the '''%''' operator
* Inline comments start with a pound symbol '''#'''
* We can map a list to a new list in just one line.  (Beat that, Java!)
* When we compute the average fruit size, we need to cast ''len(fruit_size)'', which returns an int, to a float in order to prevent integer truncation.  Note that ''sum(fruit_size)'' returns a float, even though the list contains only ints.

For some more examples, see [http://wiki.python.org/moin/SimplePrograms the Python wiki].

=== Functions ===

Define functions using the '''def''' keyword:

<source lang="python">
def my_function(name):
	print "Hello, %s!" % name

my_function("Batman")
my_function("Superman")
</source>

=== Tuples ===

Python has a special datatype called '''tuples''', which are essentially short arrays useful for storing data that is associated with each other.  They can also serve as convenient ways to assign multiple variables at once:

<source lang="python">
first_name, last_name = ("John", "Smith") # this is our tuple
</source>

Tuples also enable you to have multiple return values from a function:

<source lang="python">
def compute_length(string):
	str_len = len(string)
	if str_len < 5:
		return (str_len, "short")
	elif str_len < 40:
		return (str_len, "medium")
	else:
		return (str_len, "long")

length, description = compute_length("Four score and seven years ago")
print "The %s string is %d characters long." % (description, length)
</source>

The above example also demonstrates Python's if...elif...else conditional structure.

=== Dictionaries ===

Python has another datatype called '''dictionaries''', which are like ''maps'' in Java, ''associative arrays'' in PHP, and ''object literals'' in JavaScript (coming up soon in Module 6).  Essentially, they enable you to use strings as the keys in your data structure.

<source lang="python">
fruits_in_bowl = {
	'apple': 4,
	'banana': 2,
	'cherry': 0,
	'date': 12
}

for fruit, num in fruits_in_bowl.items():
	print "There are %d %s(s) in the bowl." % (num, fruit)
</source>

If you want only the values but not the keys from a dictionary, use '''my_dictionary.values()'''.

=== Sorting ===

Sorting in Python is frequently performed using the '''sorted''' function, which takes two arguments: a list and a function used to evaluate each item.  If the function is omitted, Python will sort the lists by value.

The following example demonstrates using an inline function, which Python calls a ''lambda''.

<source lang="python">
fruits = ['apple', 'banana', 'cherry', 'date']

# sort the fruits by string length
new_fruits = sorted(fruits, key=lambda v: len(v) )

print new_fruits # ['date', 'apple', 'banana', 'cherry']
</source>

=== Import ===

If you want to use functions from other libraries (including ones that you install using pip), use '''import''':

<source lang="python">
import time

current_time = time.localtime()
print time.strftime('%a, %d %b %Y %H:%M:%S', current_time)
</source>

If you want to pull the functions out of their namespace, you can use '''from ___ import ___''' syntax:

<source lang="python">
from time import localtime, strftime

current_time = localtime()
print strftime('%a, %d %b %Y %H:%M:%S', current_time)

# Be aware that this technique, although convenient, may cause unexpected behavior if the function names that you're pulling out of the namespace are already used for other purposes in Python.
</source>

=== File I/O ===

You can read an entire file into a variable like this:

<source lang="python">
f = open("example.txt")
file_contents = f.read()

f.close() # free up memory when we're finished with the file
</source>

You can read a file line-by-line like this:

<source lang="python">
f = open("example.txt")
for line in f:
	print "Read line: %s" % line.rstrip()

f.close()
</source>

You can write to a file like this:

<source lang="python">
# the `with` block will automatically call f.close() at the end so you don't forget:

with open("example.txt", "w") as f:
	f.write("Hello\nWorld\n")
</source>

=== Command-Line Arguments ===

Command line arguments are accessible in the variable '''sys.argv'''.

The following example shows a program that expects a filename as its argument, and it prints a usage message if the argument is not present ([http://stackoverflow.com/questions/983201/python-and-sys-argv source]).

<source lang="python">
import sys, os

if len(sys.argv) < 2:
	sys.exit("Usage: %s filename" % sys.argv[0])

filename = sys.argv[1]

if not os.path.exists(filename):
	sys.exit("Error: File '%s' not found" % sys.argv[1])
</source>

=== Object-Oriented Programming ===

You can define and use a class like this:

<source lang="python">
class Food:
	# constructor:
	def __init__(self, name):
		self.name = name
	
	@staticmethod
	def get_definition():
		return "Food is nourishment for carbon-based lifeforms."
	
	def format_name(self):
		return "Gotta love to eat " + self.name

class Fruit(Food):
	def format_name(self):
		return Food.format_name(self) + " (fruit)"

fruit = Fruit("Cherry")
print fruit.format_name()
print Food.get_definition()
</source>

This is the same example as in [[PHP#Object-Oriented Programming|the PHP guide]].

Some things to notice:
* Static methods require the '''@staticmethod''' decorator
* Non-static methods ''always'' take ''self'' as their first argument, followed by any number of additional parameters.  This can be misleading for programmers familiar with other languages, because the number of arguments you feed to the method is actually ''one less than'' the number of declared parameters.  Whenever you call a method on a class instance, that instance is implicitly fed into the explicitly-declared ''self'' parameter of the method.
*: Note that the ''self'' variable has the same purpose as the ''this'' variable in languages like PHP, Java, JavaScript, and C++.
*: Take home message is that you need to add an additional parameter, ''self'', at the beginning of any instance method.
* There is no need for a ''new'' keyword in Python.

[[Category:Module 4]]