Head First Python.md

Head First Python

The python command in the shell (assuming PATH is set) will open a REPL. If a .py file is passed as first arg to python, it will execute that file as a script. Script example (test.py):

import sys
print(sys.platform)

Then to run that script, cd into the directory containing that test.py file and python test.py

Notice that the filename as an arg requires the .py extension, but .py is omitted from import statements within scripts. Importing a module runs the code that is in that script file. The importing process happens only once per process, so a second import mymodule.py in the same process gives access to the code in mymodule.py but does not execute the mymodule.py script again.

Comments are started with # character Multiline comments use """ to start and end the comment

Variable declaration occurs at assignment, ie. There is no “var” or similar keyword. Strings are in double or single quotes. Lists are in [] and can contain any type: stuff = [1,2,'abc','xyz',3.14159]

Elements are accessed with [] notation:

stuff[0] #1
stuff[4] #3.14159
stuff[-1] #3.14159
stuff[-4] #1

Built-in functions (BIF) examples include print() and len(): len(stuff) #5

lists have methods such as append, insert, remove:

stuff.append('new thing')
stuff.insert(0, 'first thing')
stuff.remove('xyz')

iterating a list is simple:

for thing in stuff:
	print(thing)

while loops:

counter = 0
while counter < len(stuff):
	print(stuff[counter])
	counter = counter + 1

ifs:

if stuff[1] >=1 :
	print('this is true')
else:
	print('mistakes were made')

function:

def function_name(param1, param2):
	return param1 + param2
function_name(2,34)

A simple module might contain nothing but a single function def like the one above The import statement creates a namespace of the same name (the top level script has namespace __main__) so to use the module described:

import mymodule
mymodule.function_name(2,34) #36