Master Python Basics: From First Program to Syntax Essentials

First Python Program

Interactive Programming

Interactive programming does not require a script file; you write code directly in the Python interpreter.

On Linux, start it with the command python which shows a prompt like

>> Python 2.7.6 (default, Sep 9 2014, 15:04:36) [GCC 4.2.1 Compatible Apple LLVM 6.0 (clang-600.0.39)] on darwin

. On Windows, the default interactive client is installed.

Script Programming

Save code in a .py file and run it with python test.py. Example:

print "Hello, Python!"

Output: Hello, Python! Make the script executable on Unix:

#!/usr/bin/python
print "Hello, Python!"

Then run:

chmod +x test.py
./test.py

Output is the same as above.

Python Identifiers

Identifiers consist of letters, digits, and underscores, cannot start with a digit, and are case‑sensitive.

Names beginning with a single underscore (e.g., _foo) are intended for internal use; double underscores ( __foo) indicate private members; double underscores at both ends ( __foo__) denote special methods such as __init__.

Python Reserved Keywords

The following keywords cannot be used as identifiers:

and

exec

not

assert

finally

or

break

for

pass

class

from

print

continue

global

raise

def

if

return

del

import

try

elif

in

while

else

is

with

except

lambda

yield

Lines and Indentation

Python uses indentation instead of braces to define code blocks. All statements in the same block must have the same indentation level.

Incorrect indentation raises IndentationError. Consistent use of a single tab, two spaces, or four spaces is recommended.

Multi‑line Statements

Normally a newline ends a statement, but a backslash \ can continue a line. Parentheses, brackets, or braces also allow implicit line continuation.

Python String Quotes

Strings can be delimited by single ', double ", triple single ''', or triple double """ quotes. Triple quotes are useful for multi‑line strings and docstrings.

Python Comments

Single‑line comments start with #. Multi‑line comments can be written using triple quotes.

Blank Lines

Blank lines separate functions, classes, or logical sections but are not part of Python syntax. They improve readability and maintainability.

Waiting for User Input

Use raw_input("\n\nPress the enter key to exit.") to pause execution until the user presses Enter.

Multiple Statements on One Line

Separate statements with a semicolon ;. Example:

#!/usr/bin/python
import sys; x = 'runoob'; sys.stdout.write(x + '
')

Code Blocks

Statements with the same indentation form a code block (suite). Compound statements such as if, while, def, and class follow this rule.

Command‑Line Arguments

Python scripts can accept arguments; use -h to display help information.