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.
Table of Contents
The Python Tutorial¶
Python is an easy to learn, powerful programming language. It has efficient high-level data structures and a simple but effective approach to object-oriented programming. Python’s elegant syntax and dynamic typing, together with its interpreted nature, make it an ideal language for scripting and rapid application development in many areas on most platforms.
The Python interpreter and the extensive standard library are freely available in source or binary form for all major platforms from the Python web site, https://www.python.org/, and may be freely distributed. The same site also contains distributions of and pointers to many free third party Python modules, programs and tools, and additional documentation.
The Python interpreter is easily extended with new functions and data types implemented in C or C++ (or other languages callable from C). Python is also suitable as an extension language for customizable applications.
This tutorial introduces the reader informally to the basic concepts and features of the Python language and system. It helps to have a Python interpreter handy for hands-on experience, but all examples are self-contained, so the tutorial can be read off-line as well.
For a description of standard objects and modules, see The Python Standard Library . The Python Language Reference gives a more formal definition of the language. To write extensions in C or C++, read Extending and Embedding the Python Interpreter and Python/C API Reference Manual . There are also several books covering Python in depth.
This tutorial does not attempt to be comprehensive and cover every single feature, or even every commonly used feature. Instead, it introduces many of Python’s most noteworthy features, and will give you a good idea of the language’s flavor and style. After reading it, you will be able to read and write Python modules and programs, and you will be ready to learn more about the various Python library modules described in The Python Standard Library .
The Glossary is also worth going through.
- 1. Whetting Your Appetite
- 2. Using the Python Interpreter
- 2.1. Invoking the Interpreter
- 2.1.1. Argument Passing
- 2.1.2. Interactive Mode
- 2.2.1. Source Code Encoding
- 3.1. Using Python as a Calculator
- 3.1.1. Numbers
- 3.1.2. Strings
- 3.1.3. Lists
- 4.1. if Statements
- 4.2. for Statements
- 4.3. The range() Function
- 4.4. break and continue Statements, and else Clauses on Loops
- 4.5. pass Statements
- 4.6. match Statements
- 4.7. Defining Functions
- 4.8. More on Defining Functions
- 4.8.1. Default Argument Values
- 4.8.2. Keyword Arguments
- 4.8.3. Special parameters
- 4.8.3.1. Positional-or-Keyword Arguments
- 4.8.3.2. Positional-Only Parameters
- 4.8.3.3. Keyword-Only Arguments
- 4.8.3.4. Function Examples
- 4.8.3.5. Recap
- 5.1. More on Lists
- 5.1.1. Using Lists as Stacks
- 5.1.2. Using Lists as Queues
- 5.1.3. List Comprehensions
- 5.1.4. Nested List Comprehensions
- 6.1. More on Modules
- 6.1.1. Executing modules as scripts
- 6.1.2. The Module Search Path
- 6.1.3. “Compiled” Python files
- 6.4.1. Importing * From a Package
- 6.4.2. Intra-package References
- 6.4.3. Packages in Multiple Directories
- 7.1. Fancier Output Formatting
- 7.1.1. Formatted String Literals
- 7.1.2. The String format() Method
- 7.1.3. Manual String Formatting
- 7.1.4. Old string formatting
- 7.2.1. Methods of File Objects
- 7.2.2. Saving structured data with json
- 8.1. Syntax Errors
- 8.2. Exceptions
- 8.3. Handling Exceptions
- 8.4. Raising Exceptions
- 8.5. Exception Chaining
- 8.6. User-defined Exceptions
- 8.7. Defining Clean-up Actions
- 8.8. Predefined Clean-up Actions
- 8.9. Raising and Handling Multiple Unrelated Exceptions
- 8.10. Enriching Exceptions with Notes
- 9.1. A Word About Names and Objects
- 9.2. Python Scopes and Namespaces
- 9.2.1. Scopes and Namespaces Example
- 9.3.1. Class Definition Syntax
- 9.3.2. Class Objects
- 9.3.3. Instance Objects
- 9.3.4. Method Objects
- 9.3.5. Class and Instance Variables
- 9.5.1. Multiple Inheritance
- 10.1. Operating System Interface
- 10.2. File Wildcards
- 10.3. Command Line Arguments
- 10.4. Error Output Redirection and Program Termination
- 10.5. String Pattern Matching
- 10.6. Mathematics
- 10.7. Internet Access
- 10.8. Dates and Times
- 10.9. Data Compression
- 10.10. Performance Measurement
- 10.11. Quality Control
- 10.12. Batteries Included
- 11.1. Output Formatting
- 11.2. Templating
- 11.3. Working with Binary Data Record Layouts
- 11.4. Multi-threading
- 11.5. Logging
- 11.6. Weak References
- 11.7. Tools for Working with Lists
- 11.8. Decimal Floating Point Arithmetic
- 12.1. Introduction
- 12.2. Creating Virtual Environments
- 12.3. Managing Packages with pip
- 14.1. Tab Completion and History Editing
- 14.2. Alternatives to the Interactive Interpreter
- 15.1. Representation Error
- 16.1. Interactive Mode
- 16.1.1. Error Handling
- 16.1.2. Executable Python Scripts
- 16.1.3. The Interactive Startup File
- 16.1.4. The Customization Modules
- 2.1. Invoking the Interpreter