# 4.6 Python Standard Library¶

- You write Python programs by combining functions and classes (that is, custom types) that you create with preexisting functions and classes defined in modules, such as those in the Python Standard Library and other libraries.
- Avoid “reinventing the wheel.”
- A module is a file that groups related functions, data and classes.
- A package groups related modules.
- The Python Standard Library is provided with the core Python language.
- Its packages and modules contain capabilities for a wide variety of everyday programming tasks.

Some popular Python Standard Library modules
`collections`—Data structures beyond lists, tuples, dictionaries and sets.
Cryptography modules—Encrypting data for secure transmission.
`csv`—Processing comma-separated value files (like those in Excel).
`datetime`—Date and time manipulations. Also modules `time` and `calendar`.
`decimal`—Fixed-point and floating-point arithmetic, including monetary calculations.
`doctest`—Embed validation tests and expected results in docstrings for simple unit testing.
`gettext` and `locale`—Internationalization and localization modules.
`json`—JavaScript Object Notation (JSON) processing used with web services and NoSQL document databases.
`math`—Common math constants and operations.
`os`—Interacting with the operating system.
`profile`, `pstats`, `timeit`—Performance analysis.
`random`—Pseudorandom numbers.
`re`—Regular expressions for pattern matching.
`sqlite3`—SQLite relational database access.
`statistics`—Mathematical statistics functions such as `mean`, `median`, `mode` and `variance`.
`string`—String processing.
`sys`—Command-line argument processing; standard input, standard output and standard error streams.
`tkinter`—Graphical user interfaces (GUIs) and canvas-based graphics.
`turtle`—Turtle graphics.
`webbrowser`—For conveniently displaying web pages in Python apps.

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