python

Python Guide: From Zero to Hero Part 1

 

 

This is a detailed tutorial designed for coders who need to learn the Python programming language from scratch. In this course, I’ll try to highlight many of Python’s capabilities and features. Python is an easy-to-learn programming language.

Your First Program

>>> print “Hello World”
Hello World
>>>

 

On UNIX:

#!/usr/local/bin/python
print “Hello World”

 

Expressions

Expressions are the same as with other languages, as in the following:

1 + 3
1 + (3*4)
1 ** 2
’Hello’ + ’World’

 

Variables

Variables are not tied to a memory location like in C. They are dynamically typed.

a = 4 << 3
b = a * 4.5

 

if-else

# find maximum (z) of a and b
if a < b:
z = b
else:
z = a

 

elif statement

PS: There is no “switch” statement.

if a == ’+’:
op = PLUS
elif a == ’-’:
op = MINUS
elif a == ’*’:
op = MULTIPLY
else:
op = UNKNOWN

 

The while statement

while a < b:
# Do something
a = a * 2

 

The for statement

for i in [3, 4, 10, 25]:
print i
Tuples:

Tuples are like lists, but the size is fixed at the time of creation.

f = (2,3,4,5) # A tuple of integers

 

Functions

# Return a+b
def Sum(a,b):
s = a+b
return s

# Now use it
a = Sum(2,5) # a = 7

 

Files

f = open(“foo”,”w”) # Open a file for writing
g = open(“bar”,”r”) # Open a file for reading
“r” Open for reading
“w” Open for writing (truncating to zero length)
“a” Open for append
“r+” Open for read/write (updates)
“w+” Open for read/write (with truncation to zero length)

 

Examples of string processing functions

string.atof(s) # Convert to float
string.atoi(s) # Convert to integer
string.atol(s) # Convert to long
string.count(s,pattern) # Count occurrences of pattern in s.

Operating System Services

First, when we talk about operating system, we mean the management of computer hardware and software resources and providing common services for computer programs.

Python has the power to easily manipulate the system calls, Operating environment, Processes, Timers, Signal handling, Error reporting etc…

 

Let’s start with the Environment Variables:

user = os.environ[’USER’]

os.environ[’PATH’] = “/bin:/usr/bin”

 

Working directory

os.chdir(path)# Change current working directoryos.getcwd()# Get it

 

Users and groups

os.getegid()# Get effective group idos.geteuid()# Get effective user idos.getgid()# Get group idos.getuid()# Get user idos.setgid(gid)# Set group idos.setuid(uid)# Set user id

 

Process

os.fork()# Create a child processos.execv(path,args)# Execute a process os.execve(path, args, env)os.execvp(path, args)# Execute process, use default path os.execvpe(path,args, env)os.wait([pid)]# Wait for child processos.waitpid(pid,options)# Wait for change in state of childos.system(command)# Execute a system commandos._exit(n)# Exit immediately with status n

Regular Expressions in PYTHON

Definition

Regular expressions are patterns used to match character combinations. Take a look at given below table:

Regular Expressions

Description

foo.*# Matches any string starting with food*# Match any number decimal digits[a-zA-Z]+# Match a sequence of one or more letterstextMatch literal text.Match any character except newline^Match the start of a string$Match the end of a string*Match 0 or more repetitions+Match 1 or more repetitions?Match 0 or 1 repetitions *? Match 0 or more, few as possible+?Match 1 or more, few as possible{m,n}Match m to n repetitions{m,n}?Match m to n repetitions, few as possible[…]Match a set of characters[^…]Match characters not in setA | BMatch A or B (…) Match regex in parenthesis as a groupnumberMatches text matched by previous groupAMatches start of stringbMatches empty string at beginning or end of wordBMatches empty string not at begin or end of worddMatches any decimal digitDMatches any non-digitsMatches any whitespaceSMatches any non-whitespacewMatches any alphanumeric characterWMatches characters not inw ZMatch at end of string.\Literal backslash