python 学习笔记（4）

1. Expression in parentheses (), square bracket [], and curly braces {} can span multiple lines.
2. A backslash (\) at the end of the line denotes continuation to the next line. This is an old rule and is NOT recommended as it is error-prone.

A compound statement, such as def (function definition), while-loop, begins with a header line terminated with a colon (:); followed by the indented body block. Python does not specify how much indentation to use, but all statements of the body block must start at the same distance from the right margin. 

In Python, 0, None and empty value (such as empty string ‘‘, "", empty list [], empty tuple (), empty dictionary {}) are treated as False; anything else are treated as True. Booleans can also act as integers in arithmetic operations with 1 for True and 0 for False. 

Python is dynamic typed. It associates types with objects, instead of variables. That is, a variable does not have a fixed type and can be assigned an object of any type. A variable simply provides a reference to an object.

You do not need to declare a variable. A variable is created automatically when a value is first assigned, which link the variable to the object. You can use built-in function type(var_name) to get the object type referenced by a variable.

Python does not support increment (++) and decrement (--) operators (as in C/C++/Java). You need to use i = i + 1 or i += 1 for increment.

import module_name statement: To refer to an attribute, use module_name.attr_name.

from module_name import attr_name statement: You can use attr_name directly without the module_name.

from module_name import * statement: Import all attributes of the module.