Python OG

Using the Python Interpreter

2.1 Invoking the Interpreter

Python path

On Unix-like machines you can find the executable file under /usr/local/bin/

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$ file /usr/local/bin/python3.9
/usr/local/bin/python3.9: Mach-O 64-bit executable x86_64

Since the choice of the directory where the interpreter lives is an installation option, other places are possible

Exit Python

  • enter EOF character (Ctrl-D on Unix, Ctrl-Z on Windows)

  • exit via cmd quit()

  • if exit gracefully, exit status will be 0

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    $ python3.9
    Python 3.9.1 (default, Dec 29 2020, 08:52:17) 
    [Clang 12.0.0 (clang-1200.0.32.28)] on darwin
    Type "help", "copyright", "credits" or "license" for more information.
    >>> quit()
    $ echo $?
    0

    Editing

  • If Ctrl-P have beeps, line-edit is allowed

  • 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.

  • TTY : stand for TeleTYpewriter

    PTS : stand for Pseudo-TeletypeS

    more

  • simple cmd : python -c command [arg] ..., command is suggested to be quoted with single quotes

  • use module : python -m module [arg] ...

  • enter interactive mode after executing a script file :

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    $ python3 test.py
    hello
    $ python3 -i test.py
    hello
    >>>

  • Command line and environment

2.1.1. Argument Passing

The script name and additional arguments are turned into a list of string stored in variable sys.argv

  • The length of the list is at least one
condition argv[0]
no script and no arguments ‘’ (void string)
the script name is given as '-'(stdin) ‘_’
-c command is used ‘-c’
-m module is used 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.
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# source of test.py
import sys
for i in sys.argv:
  print(type(i) , i)
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$ python3 test.py hello world
<class 'str'> test.py
<class 'str'> hello
<class 'str'> world

2.1.2. Interactive Mode

Interactive Mode

2.2. The Interpreter and Its Environment

2.2.1. Source Code Encoding

  • By default, Python source code use UTF-8 encoding

  • All Python standard libraries use ASCII, a convention that any portable code should follow

  • 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:

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    # -*- coding: encoding -*-

    for example:

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    # -*- coding: cp1252 -*-

    the first or second line must match the regular expression “coding[:=]\s*([-\w.]+)”

    so # -*- encoding: xxxxxx -*- also works

  • If the source code starts with a UNIX “shebang” line, encoding massage should be the second line

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    #!/usr/bin/env python3
    # -*- coding: cp1252 -*-

  • What does a “shebang” line do? —- make a python script a shell script

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    print("Hello World")
    # source of test.py
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    $ ./test.py
    permission denied: ./test.py
    $ ls -l test.py
    -rw-r--r--  1 ******  staff  21 Feb  2  **:** test.py
    $ chmod +x test.py
    $ ls -l test.py
    -rwxr-xr-x  1 ******  staff  21 Feb  2  **:** test.py
    $ ./test.py
    ./test.py: line 1: syntax error near unexpected token `'Hello World''
    ./test.py: line 1: `print('Hello World')'
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    #! /usr/bin/env python3
    print("Hello World")
    # source of test.py
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    $ ./test.py
    Hello World

  • NOTE that the declaration does NOT change the encoding acutually

    it only applies to how Python reads the source code. It doesn’t apply to executing that code, so not to how printing, opening files, or any other I/O operations translate between bytes and Unicode.

usually used to solve UnicodeEncodeError