学习Python的利器：内置函数dir()和help()

（1）内置函数dir()用来查看对象的成员。在Python中所有的一切都是对象，除了整数、实数、复数、字符串、列表、元组、字典、集合等等，还有range对象、enumerate对象、zip对象、filter对象、map对象等等，函数也是对象，类也是对象，模块也是对象。这样的话，dir()的用武之地就大了。

>>> dir(3)  #查看整数类型的成员，这里省略了输出结果

>>> dir('a') #查看字符串类型的成员

>>> import math

>>> dir(math) #查看math模块的成员

>>> def demo():pass #定义一个空函数

>>> dir(demo) #查看函数的成员

>>> class Demo: #定义一个类

     def __init__(self):pass

     def methodA(self):pass

     def methodB(self):pass

>>> dir(Demo) #查看类的成员

['__class__', '__delattr__', '__dict__', '__dir__', '__doc__', '__eq__', '__format__', '__ge__', '__getattribute__', '__gt__', '__hash__', '__init__', '__init_subclass__', '__le__', '__lt__', '__module__', '__ne__', '__new__', '__reduce__', '__reduce_ex__', '__repr__', '__setattr__', '__sizeof__', '__str__', '__subclasshook__', '__weakref__', 'methodA', 'methodB']

>>> d = Demo() #实例化一个对象

>>> dir(d) #查看对象成员

['__class__', '__delattr__', '__dict__', '__dir__', '__doc__', '__eq__', '__format__', '__ge__', '__getattribute__', '__gt__', '__hash__', '__init__', '__init_subclass__', '__le__', '__lt__', '__module__', '__ne__', '__new__', '__reduce__', '__reduce_ex__', '__repr__', '__setattr__', '__sizeof__', '__str__', '__subclasshook__', '__weakref__', 'methodA', 'methodB']

（2）内置函数help()用来返回对象的帮助信息，尤其常用来查看函数或对象方法的帮助信息。除此之外，help()还可以做很多事，例如查看模块的帮助信息，以及Python关键字和运算符的信息。

>>> help(sum)  #查看内置函数的帮助文档

Help on built-in function sum in module builtins:

sum(iterable, start=0, /)

    Return the sum of a 'start' value (default: 0) plus an iterable of numbers

   

    When the iterable is empty, return the start value.

    This function is intended specifically for use with numeric values and may

    reject non-numeric types.

>>> help(3)  #查看整数类型的帮助文档

Help on int object:

class int(object)

 |  int(x=0) -> integer

 |  int(x, base=10) -> integer

...（其他输出结果略去）

>>> help('math')  #查看标准库的帮助文档，注意要加引号

Help on built-in module math:

NAME

    math

DESCRIPTION

    This module is always available.  It provides access to the

    mathematical functions defined by the C standard.

...（其他输出结果略去）

>>> help('')  #查看字符串类型的帮助文档

Help on class str in module builtins:

class str(object)

 |  str(object='') -> str

 |  str(bytes_or_buffer[, encoding[, errors]]) -> str

...（其他输出结果略去）

>>> import math

>>> help(math.sin)  #查看标准库函数的帮助文档

Help on built-in function sin in module math:

sin(...)

    sin(x)

   

    Return the sine of x (measured in radians).

>>> help()   #进入帮助环境

Welcome to Python 3.6's help utility!

If this is your first time using Python, you should definitely check out

the tutorial on the Internet at http://docs.python.org/3.6/tutorial/.

Enter the name of any module, keyword, or topic to get help on writing

Python programs and using Python modules.  To quit this help utility and

return to the interpreter, just type "quit".

To get a list of available modules, keywords, symbols, or topics, type

"modules", "keywords", "symbols", or "topics".  Each module also comes

with a one-line summary of what it does; to list the modules whose name

or summary contain a given string such as "spam", type "modules spam".

help>

>>> help('in')  #查看Python关键字的帮助文档

Membership test operations

**************************

The operators "in" and "not in" test for membership.  "x in s"

evaluates to true if *x* is a member of *s*, and false otherwise.  "x

not in s" returns the negation of "x in s".  All built-in sequences

and set types support this as well as dictionary, for which "in" tests

whether the dictionary has a given key. For container types such as

list, tuple, set, frozenset, dict, or collections.deque, the

expression "x in y" is equivalent to "any(x is e or x == e for e in

y)".

...（其他输出结果略去）

>>> help('with')  #查看Python关键字with的帮助文档

The "with" statement

********************

The "with" statement is used to wrap the execution of a block with

methods defined by a context manager (see p With Statement

Context Managers). This allows common "try"..."except"..."finally"

usage patterns to be encapsulated for convenient reuse.

   with_stmt ::= "with" with_item ("," with_item)* ":" suite

   with_item ::= expression ["as" target]

...（其他输出结果略去）

>>> help('for')  #查看Python关键字for的帮助文档

The "for" statement

*******************

The "for" statement is used to iterate over the elements of a sequence

(such as a string, tuple or list) or other iterable object:

   for_stmt ::= "for" target_list "in" expression_list ":" suite

                ["else" ":" suite]

...（其他输出结果略去）

>>> help('+')  #查看所有运算符的帮助文档

Operator precedence

*******************

The following table summarizes the operator precedence in Python, from

lowest precedence (least binding) to highest precedence (most

binding).  Operators in the same box have the same precedence.  Unless

the syntax is explicitly given, operators are binary.  Operators in

the same box group left to right (except for exponentiation, which

groups from right to left).

...（其他输出结果略去）

那么问题来了，如果我们自己设计了函数或类或模块，如何能够通过help()给出帮助文档呢？答案是写注释。

>>> def add(x, y):  #定义函数，编写注释

     '''Accept integers x and y, return x+y'''

     return x+y

>>> help(add)  #查看自定义函数的帮助文档

Help on function add in module __main__:

add(x, y)

    Accept integers x and y, return x+y

>>> class T:

     '''This is a test.'''

     pass

>>> help(T)  #查看自定义类的帮助文档

Help on class T in module __main__:

class T(builtins.object)

 |  This is a test.

 | 

 |  Data descriptors defined here:

 | 

 |  __dict__

 |      dictionary for instance variables (if defined)

 | 

 |  __weakref__

 |      list of weak references to the object (if defined)