Basic Grammar

Data types

• Integer: Python can handle integers of arbitrary size (Python 2.x had two types of integers, int and long, but this distinction is not very meaningful for Python, so in Python 3.x integers are now only int), and supports binary (e.g. 0b100, which converts to decimal as 4), octal (e.g. 0o100, which translates to 64 in decimal), decimal (100) and hexadecimal (0x100, which translates to 256 in decimal) representations.

• Floating point: Floating point numbers are also known as decimals, so called because the position of the decimal point of a floating point number is variable when expressed in scientific notation, and floating point numbers support scientific notation (e.g. 1.23456e2) in addition to mathematical writing (e.g. 123.456).

• Strings: Strings are arbitrary text enclosed in single or double quotes, such as 'hello'' and "hello"`, strings are also available in raw string representation, byte string representation, Unicode string representation and can be written in multi-line form (starting with three single or three double quotes and ending with three single or three double quotes).

• Boolean: Boolean values have only two values, True and False, either True or False. In Python, Boolean values can be expressed directly as True and False (please note the case), or they can be calculated by Boolean operations (e.g. 3 < 5 produces the Boolean value True, while 2 == 1 will produce the Boolean value False).

• The complex type: shaped as 3 + 5j is the same as the mathematical representation of a complex number, the only difference being that the i in the imaginary part is replaced by j. In fact, this type is not commonly used, so it is good to know about it.

Variable naming

Hard and fast rules:

• Variable names consist of letters (broad Unicode characters, excluding special characters), numbers and underscores; numbers cannot begin.
• Case sensitive (an upper case a and a lower case A are two different variables).
• Do not conflict with keywords (words with special meanings, covered later) and system reserved words (e.g. names of functions, modules, etc.).

PEP 8 requires:

• Spell words in lowercase letters, with multiple words connected by underscores.
• Protected instance attributes begin with a single underscore (covered later).
• Private instance attributes begin with two underscores (more on this later).

Using type() to check the type of a variable

The built-in functions in Python perform conversions on variable types.

• int(): converts a numeric value or string to an integer, you can specify the binary.
• float(): converts a string to a floating point number.
• str(): converts a specified object to string form, with the possibility of specifying an encoding.
• chr(): converts an integer to the string corresponding to that encoding (one character).
• ord(): converts a string (a character) into the corresponding encoding (an integer).

Operators

operator	description
[] [:]	subscript, slice
*	exponentiation
~ + -	Inverse by bit, plus or minus sign
* / % /	multiply, divide, modulo, integer division
+ -	add, subtract
> <<	shift right, shift left
&	by bit with
^ `	`
<= < > >=	less-than-equal, less-than, greater-than, greater-than-equal
== `! =``	equal to, not equal to
is is not	identity operator
in not in	member operators
not or and	logical operators
= += -= *= /= %= //= **= &= `	= ^= >>= <<=`

• In practical development, if you are confused about the precedence of operators, you can use parentheses to ensure that the operations are executed in order. *

Keyword

keyword	description
and	Logical operators.
as	Create an alias.
assert	Used for debugging.
break	exit the loop.
class	Defines a class.
continue	Continue to the next iteration of the loop.
def	Defines a function.
del	Delete the object.
elif	Used in conditional statements, equivalent to else if.
else	Used in conditional statements.
except	Handles exceptions and how to execute them if they occur.
False	Boolean value, the result of a comparison operation.
finally	Handles exceptions, and executes a piece of code whether or not an exception exists.
for	Creates a for loop.
from	Import a specific part of the module.
global	Declare global variables.
if	Write a conditional statement.
import	Importing a module.
in	Check if a value exists in a collection such as a list, tuple, etc.
is	Tests if two variables are equal.
lambda	Create anonymous functions.
None	Indicates a null value.
nonlocal	Declares a non-local variable.
not	The logical operator.
or	The logical operators.
pass	null statement, a statement that does nothing.
raise	Raises an exception.
return	Quit the function and return the value.
True	Boolean, the result of a comparison operation.
try	Write try... ...except statements.
while	Create a while loop.
with	is used to simplify exception handling.
yield	ends the function and returns the generator.

Comments

• Single line comments: those starting with # and a space

• Multi-line comments: three quotes at the beginning and three quotes at the end

"""
First Python program - hello, world!
A tribute to the great Dennis M. Ritchie
Version: 0.1
Author: ``Luo Hao
"""
# hello world!
print('hello, world!')
print("hello, world!")