Execute python program in shell

2. Using the Python Interpreter¶

The Python interpreter is usually installed as /usr/local/bin/python3.11 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:

to the shell. 1 Since 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 python3.11 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() .

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 beeps, 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.

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.

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.

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.

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.

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:

$ python3.11 Python 3.11 (default, April 4 2021, 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:

>>> 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 simultaneously 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.

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:

where encoding is one of the valid codecs supported by Python.

For example, to declare that Windows-1252 encoding is to be used, the first line of your source code file should be:

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:

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

On Unix, the Python 3.x interpreter is by default not installed with the executable named python , so that it does not conflict with a simultaneously installed Python 2.x executable.

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How to Run a Python Script via a File or the Shell

If you can’t execute or run a Python script, then programming is pointless. When you run a Python script, the interpreter converts a Python program into something that that the computer can understand. Executing a Python program can be done in two ways: calling the Python interpreter with a shebang line, and using the interactive Python shell.

Run a Python Script as a File

Generally programmers write stand alone scripts, that are independent to live environments. Then they save it with a «.py» extension, which indicates to the operating system and programmer that the file is actually a Python program. After the interpreter is invoked, it reads and interprets the file. The way Python scripts are run on Windows versus Unix based operating systems is very different. We’ll show you the difference, and how to run a Python script on Windows and Unix platforms.

Run a Python script under Windows with the Command Prompt

Windows users must pass the path of the program as an argument to the Python interpreter. Such as follows:

[shell]
C:\Python27\python.exe C:\Users\Username\Desktop\my_python_script.py
[/shell]

Note that you must use the full path of the Python interpreter. If you want to simply type python.exe C:\Users\Username\Desktop\my_python_script.py you must add python.exe to your PATH environmental variable. To do this, checkout the adding Python to the PATH environment article..

Window’s python.exe vs pythonw.exe

Note that Windows comes with two Python executables — python.exe and pythonw.exe . If you want a terminal to pop-up when you run your script, use python.exe However if you don’t want any terminal pop-up, use pythonw.exe . pythonw.exe is typically used for GUI programs, where you only want to display your program, not the terminal.

Run a Python Script Under Mac, Linux, BSD, Unix, etc

On platforms like Mac, BSD or Linux (Unix) you can put a «shebang» line as first line of the program which indicates the location of the Python interpreter on the hard drive. It’s in the following format:

[python]
#!/path/to/interpreter
[/python]

A common shebang line used for the Python interpreter is as follows:

[python]
#!/usr/bin/env python
[/python]

You must then make the script executable, using the following command:

[shell]
chmod +x my_python_script.py
[/shell]

Unlike Windows, the Python interpreter is typically already in the $PATH environmental variable, so adding it is un-necessary.

You can then run a program by invoking the Python interpreter manually as follows:

[python]
python firstprogram.py
[/python]

Python Execution with the Shell (Live Interpreter)

Assuming that you already have Python installed and running well (if you’re getting an error, see this post), open the terminal or console and type ‘python’ and hit the ‘Enter’ key. You will then be directed immediately to the Python live interpreter. Your screen will display a message something like:

[python]
user@hostname:~ python
Python 3.3.0 (default, Nov 23 2012, 10:26:01)
[GCC 4.2.1 Compatible Apple Clang 4.1 ((tags/Apple/clang-421.11.66))] on darwin
Type «help», «copyright», «credits» or «license» for more information.
>>>
[/python]

The Python programmer should keep in mind one thing: that while working with the live interpreter, everything is read and interpreted in real-time. For example loops iterate immediately, unless they are part of function. So it requires some mental planning. Using the Python shell is typically used to execute code interactively. If you want to run a Python script from the interpreter, you must either import it or call the Python executable.

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