再读 Python Language Reference

https://docs.python.org/3.6/reference/ 重读笔记。

Lexical analysis

  • 在Python源文件中,空格将代码分割成token(s)。物理换行符(UNIX下的\n)对 Python程序没有实际意义,Python程序认定的是 NEWLINE

  • 在Python源文件中,一个Tab键在Python解释器解释的时候会被替换成8个空格的缩进, 而缩进对于Python有非常重要的意义。所以推荐在自己用的解释器里都把tab键替换成 4个空格(或者自己想要的空格数)。

>>> def foo():
...     print("hello")  # 1 tab here
...     print("world")  # 4 spaces here
  File "<stdin>", line 3
    print("world")
                 ^
IndentationError: unindent does not match any outer indentation level
  • NEWLINE

    • 显式join:使用\
    >>> if True\
    ... and False:
    ...     print("False")
    ...
    >>>
    
    • 隐式join:在小括号,中括号,大括号中的行都会被合并成逻辑行(只有一个NEWLINE)
    >>> (
        ... "hello"
        ... "world"
        ... )
    'helloworld'
    >>>
    

Data model

Python中每个对象都有自己的id,类型和值。

  • Python中的对象从对象的值的可变性上分为两类:

    • mutable,例如 dict, list
    • immutable,例如 str, tuple
  • 从理论上来讲,副作用例如打开一个文件,当此文件没有引用时是会被Python垃圾回收 的,但是由于gc无法做出保证(例如Python开启了DEBUG模式,所有的对象都会被一个 双链表链着,这个时候引用计数就不为0),所以最好还是自己显式关闭。

Execution model

  • Python程序由 code blocks 组成,block是Python执行代码的最小单元,block分为:

    • module
    • function body
    • class definition
    • 交互模式下每次输入的命令都是一个block
    • 每一个script file
    • $ python -c 'print("hello")' 中的命令也是一个block
    • 传递给 eval()exec() 的字符串也是一个block,但是这两个函数作用域规则有些特殊,会跳过LEGB中的E,也就是闭包。

每个block都在一个 execution frame 里执行。

  • name binding

    • 给函数传参数
    • import语句
    • 类和函数定义
    • 赋值语句
    • for语句的后面一个 for i in ...
    • with语句的as后面 with open(...) as ...
    • except后面的as
    • from ... import * 绑定所有非下划线开头的名字到当前命名空间
    • del 也是,虽然它的作用是删除绑定

只要出现了上述的binding,Python就认为这个变量名所指向的变量在当前的block里,所以:

In [1]: foo = "hello"

In [2]: def print_foo():
   ...:     print(foo)
   ...:     foo = "world"
   ...:

In [3]: print_foo()
---------------------------------------------------------------------------
UnboundLocalError                         Traceback (most recent call last)
<ipython-input-3-c80be14760b6> in <module>()
----> 1 print_foo()

<ipython-input-2-afc4cd9aff75> in print_foo()
      1 def print_foo():
----> 2     print(foo)
      3     foo = "world"
      4

UnboundLocalError: local variable 'foo' referenced before assignment

In [4]:
  • "The global statement has the same scope as a name binding operation in the same block. If the nearest enclosing scope for a free variable contains a global statement, the free variable is treated as a global."

没看懂,来自:https://docs.python.org/3.6/reference/executionmodel.html

  • 类定义的变量作用域局限于类内,而不会扩展至方法和表达式,generator里,所以:
In [1]: class Foo:
   ...:     hello = "hello"
   ...:     def foo(self):
   ...:         print(hello)
   ...:

In [2]: Foo().foo()
---------------------------------------------------------------------------
NameError                                 Traceback (most recent call last)
<ipython-input-2-6c4f5adc4d1e> in <module>()
----> 1 Foo().foo()

<ipython-input-1-029db0d572c9> in foo(self)
      2     hello = "hello"
      3     def foo(self):
----> 4         print(hello)
      5

NameError: name 'hello' is not defined

In [3]: class Foo:
   ...:     a = 32
   ...:     b = [a + i for i in range(10)]
   ...:
---------------------------------------------------------------------------
NameError                                 Traceback (most recent call last)
<ipython-input-3-395876c62295> in <module>()
----> 1 class Foo:
      2     a = 32
      3     b = [a + i for i in range(10)]
      4

<ipython-input-3-395876c62295> in Foo()
      1 class Foo:
      2     a = 32
----> 3     b = [a + i for i in range(10)]
      4

<ipython-input-3-395876c62295> in <listcomp>(.0)
      1 class Foo:
      2     a = 32
----> 3     b = [a + i for i in range(10)]
      4

NameError: name 'a' is not defined

In [4]:
  • 闭包内的变量(free variables)的值是在运行时确定的:
In [1]: foo = "hello"

In [2]: def f():
   ...:     print(foo)
   ...:

In [3]: f()
hello

In [4]: foo = "world"

In [5]: f()
world

In [6]:

The import system

  • import 语句包含两个部分

    • __import__ 搜寻指定名称的包
    • 然后将 __import__ 返回值在语句所在的scope里binding成那个名字
  • 导入的时候,会根据点号依次导入,而且会首先搜寻 sys.modules,例如:

In [1]: import sys

In [2]: sys.modules['tornado']
---------------------------------------------------------------------------
KeyError                                  Traceback (most recent call last)
<ipython-input-2-36f3d84239fe> in <module>()
----> 1 sys.modules['tornado']

KeyError: 'tornado'

In [3]: sys.modules['tornado.web']
---------------------------------------------------------------------------
KeyError                                  Traceback (most recent call last)
<ipython-input-3-f1a34b2734fb> in <module>()
----> 1 sys.modules['tornado.web']

KeyError: 'tornado.web'

In [4]: import tornado.web

In [5]: tornado
Out[5]: <module 'tornado' from '/usr/local/lib/python3.5/dist-packages/tornado/__init__.py'>

In [6]: tornado.web
Out[6]: <module 'tornado.web' from '/usr/local/lib/python3.5/dist-packages/tornado/web.py'>

In [7]: sys.modules['tornado']
Out[7]: <module 'tornado' from '/usr/local/lib/python3.5/dist-packages/tornado/__init__.py'>

In [8]: sys.modules['tornado.web']
Out[8]: <module 'tornado.web' from '/usr/local/lib/python3.5/dist-packages/tornado/web.py'>

In [9]:

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