Using the Python Interpreter
2.1. Invoking the Interpreter
The Python interpreter is usually installed as /usr/local/bin/python3.12 on those machines where it is available; putting /usr/local/bin in your Unix shell’s search path makes it possible to start it by typing the command:
python3.12
to the shell. 1Since the choice of the directory where the interpreter lives is an installation option, other places are possible; check with your local Python guru or system administrator. (E.g., /usr/local/python is a popular alternative location.)
On Windows machines where you have installed Python from the Microsoft Store, the python3.12 command will be available. If you have the py.exe launcher installed, you can use the py command. See Excursus: Setting environment variables for other ways to launch Python.
Typing an end-of-file character (Control-D on Unix, Control-Z on Windows) at the primary prompt causes the interpreter to exit with a zero exit status. If that doesn’t work, you can exit the interpreter by typing the following command: quit().
在 Python 解释器的提示词中输入 End-of-File 字符（如 Unix 机上的 Ctrl+D ，或 Windows 机上的 Ctrl+Z）就可以让解释器以 0 状态退出。或者调用
quit()函数也行。
The interpreter’s line-editing features include interactive editing, history substitution and code completion on systems that support the GNU Readline library. Perhaps the quickest check to see whether command line editing is supported is typing Control-P to the first Python prompt you get. If it beeps2, you have command line editing; see Appendix Interactive Input Editing and History Substitution for an introduction to the keys. If nothing appears to happen, or if ^P is echoed, command line editing isn’t available; you’ll only be able to use backspace to remove characters from the current line.
The interpreter operates somewhat like the Unix shell: when called with standard input connected to a tty device, it reads and executes commands interactively; when called with a file name argument or with a file as standard input, it reads and executes a script from that file.
解释器的运行模式与Unix shell 类似，
- 当被连接到 tty3 设备的标准输入调用时，它会交互式地读取并执行输入的命令；
- 当被文件名作为参数或文件作为标准输入调用时，它会从那个文件执行一个脚本。
A second way of starting the interpreter is python -c command [arg] ..., which executes the statement (s) in command, analogous to the shell’s -c option. Since Python statements often contain spaces or other characters that are special to the shell, it is usually advised to quote command in its entirety.
另一种启动解释器的方式是使用命令
python -c command [arg] ...，这将执行command 中的语句，类似于shell 中的 -c 选项4。另外，由于 Python 语句通常含有空格或 shell 中有特殊用途的字符，因此建议用
""引用 command 中的全部内容。
Some Python modules are also useful as scripts. These can be invoked using python -m module [arg] ..., which executes the source file for module as if you had spelled out its full name on the command line.
一些 Python 模块可以作为脚本使用，我们可以使用
python -m module [arg] ...这条命令调用它们，它会执行 module 中指定的源文件，就像在命令行中拼出其全名一样。
When a script file is used, it is sometimes useful to be able to run the script and enter interactive mode afterwards. This can be done by passing -i before the script.
使用脚本文件时，有时需要在脚本运行后进入交互模式，这可以通过在脚本前传递
-i参数来实现。
All command line options are described in Command line and environment.
2.1.1. Argument Passing
When known to the interpreter, the script name and additional arguments thereafter are turned into a list of strings and assigned to the argv variable in the sys module. You can access this list by executing import sys. The length of the list is at least one; when no script and no arguments are given, sys.argv[0] is an empty string. When the script name is given as '-' (meaning standard input), sys.argv[0] is set to '-'. When -c command is used, sys.argv[0] is set to '-c'. When -m module is used, sys.argv[0] is set to the full name of the located module. Options found after -c command or -m module are not consumed by the Python interpreter’s option processing but left in sys.argv for the command or module to handle.
参数传递
当解释器知道脚本名称及其后的附加参数时，会将其转换为字符串列表，并赋值给
sys模块中的argv变量：
- 可以通过
import sys引入sys模块来访问这个列表- 列表的长度至少是 1 ；如果没有指定脚本或参数，那么
sys.argv[0]是一个空字符串- 当脚本名字被指定为
-（意味着标准输入），sys.argv[0]也会被设置为-；当使用了-c command选项，sys.argv[0]被设置为-c；当使用-m module选项，就被设置为定位到的模块的全称- 而在
-c command或-m module之后的选项不会被 Python 解释器使用，而是保留在sys.argv列表中交给模块自己处理
2.1.2. Interactive Mode
When commands are read from a tty, the interpreter is said to be in interactive mode. In this mode it prompts for the next command with the primary prompt, usually three greater-than signs (>>>); for continuation lines it prompts with the secondary prompt, by default three dots (...). The interpreter prints a welcome message stating its version number and a copyright notice before printing the first prompt:
当从tty 中读取到命令
python3.12后，解释器会进入 交互模式 ：
- 此模式下会通过 主要提示符（通常是三个大于号
>>>）提示用户输入下一个命令- 对于续行，则会使用 辅助提示（通常是三个句点
...）解释器会在开始阶段，打印欢迎信息，包括Python 版本号、版权通知等信息。
$ python3.12
Python 3.12 (default, April 4 2022, 09:25:04)
[GCC 10.2.0] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>>
Continuation lines are needed when entering a multi-line construct. As an example, take a look at this if statement:
续行发生在键入一个多行的语句块（这类块通常以
:结尾）时，如if-else 块，此时键入 if 行后直接回车，就会进入续行模式，可以看到提示符发生变化（之后的语句不要忘记 Python 的缩进问题）。在完成多行的语句块编写后，连续两次回车后就可以退出多行语句块的编写。
>>> the_world_is_flat = True
>>> if the_world_is_flat:
... print("Be careful not to fall off!")
...
Be careful not to fall off!
For more on interactive mode, see Interactive Mode.
2.2. The Interpreter and Its Environment
2.2.1. Source Code Encoding
By default, Python source files are treated as encoded in UTF-8. In that encoding, characters of most languages in the world can be used simultaneously5 in string literals, identifiers and comments — although the standard library only uses ASCII characters for identifiers, a convention that any portable code should follow. To display all these characters properly, your editor must recognize that the file is UTF-8, and it must use a font that supports all the characters in the file.
默认情况下 Python 源文件使用 UTF-8 编码，在这种编码中，世界上大多数语言的字符都可以在字符串字面量、标识符和注释中同时使用，尽管标准库只在标识符中使用 ASCII 字符——这是任何可移植代码都应遵循的惯例。
因此，要成功显示UTF-8 编码，需要编辑器能够识别、字体能够支持它。
To declare an encoding other than the default one, a special comment line should be added as the first line of the file. The syntax is as follows:
为了标明当前文件的编码方式（只要不是 UTF-8 都要表明），就需要在文件第一行用特殊的注释注明。
# -*- coding: encoding -*-where encoding is one of the valid codecs supported by Python.
这里 encoding 应当是 Python 支持的合法编码方式（默认为 UTF-8）。
For example, to declare that Windows-1252 encoding is to be used, the first line of your source code file should be:
# -*- coding: cp1252 -*-One exception to the first line rule is when the source code starts with a UNIX “shebang” line. In this case, the encoding declaration should be added as the second line of the file. For example:
一般而言 Python 脚本的第一行都以编码方式开始，除非你使用 Unix 系统——如果 Python 脚本可以像shell 脚本一样直接执行，那么第一行要注明解释器的路径、版本，并且一定要以
#!开头。
#!/usr/bin/env python3
# -*- coding: cp1252 -*-