Checklist for
Python libraries APIs

Useful checklist for build good Python libraries APIs. Based on "How to make a good library API" PyCon 2017 talk.

Checkboxes below are saved in localStorage. You can mark what you have and come back later.


  • Write sample client code in the README
    E.g. Pendulum's README starts with sample code
  • Reduce boilerplate code: count the number of lines up to first really useful API call
    E.g. urllib2 has lots of boilerplate code to do an HTTP request compared to Requests library
  • Document use cases
    E.g. python-social-auth document common use-cases
  • Practice Progressive disclosure: have sane and smart defaults
      • Have default arguments and decide their values according to the most common use cases
      • Have default settings and decide their values according to the most common use cases
      • Decide argument order by listing the most used ones first and grouping related ones together
        E.g. JavaScript history.pushState has bad default argument order: state, title, URL. Most API clients just want to add a URL to the history and they're forced to specify state and title.
      • Don't require copy and pasting of code snippets
  • Avoid cumbersome inputs:
      • Check if parameter name is misleading
        E.g. In Scrapy 1.2, the send method accepts a "to" parameter as a list of strings. If the client passes a single string, the method will iterate over the string, trying to send emails to each character of it. Scrapy 1.3 fixed this by accepting both a single string and a list of strings
      • Check if the user is instantiating something just to call your API. If so, consider accepting the wrapped value
        E.g. a function that only accepts file-like objects will force clients to use StringIO if they want to pass strings.
      • Check if it's possible to replace a custom type with a built-in one, or support both
  • Respect the Principle of least astonishment: "If a feature has a high astonishment factor, it may be necessary to redesign it"
      • The default behavior does what clients expect?
      • The default behavior matches client expectations about performance, side-effects, safety, security, restrictions, etc.?
  • Create abstractions
      • Draw away from the physical HOW nature of the solution to focus on the WHAT nature
        E.g. to make a simple Celery task, devs don't need to worry much about task queues, workers, message brokers, serialization, etc. They just need to use the @app.task decorator. The API is focused on the task definition, not on the tasks inner-workings.
  • Be Pythonic
      • Strive for common Python idioms, try to make API calls look like built-in Python API calls
        E.g. in Python 2, ConfigParser.get receives a section and option. This isn't natural in Python since get of dict receives key and default value. In Python 3, this was fixed by introducing a dict-like interface to ConfigParser


  • Naming: is there naming consistency?
      • Order of terms: string_encode vs decode_string.
      • Abbreviations: activate_prev vs fetch_previous, bin2hex vs strtolower.
      • With or without underscore: gettype vs get_class
      • Plurals: values_list vs value_list
      • Negatives: button.enabled == True vs button.disabled == True
  • Emptiness: is there consistency in what means the empty value? Is it the best representation?
      • Decide what will be used as empty: None, False, [], '', 0, etc.
      • Be careful with unexpected falsy values: bool(datetime.time(0)) == False in Python < 3.5
  • Parameters: is there consistency in the order of parameters?
    E.g. datetime.datetime(year, month, day, minute, second, microsecond) vs datetime.timedelta(days, seconds, microseconds, milliseconds, minutes, hours, weeks)
  • Behavior: are similar behaviors represented similar and different behaviors represented differently? Asymmetry of behavior should reflect in asymmetry of form
    E.g. Good: numbers.sort() in-place vs sorted(numbers) not-in-place.


  • Reduce integration discontinuities
      • Check if classes have a single responsibility. It not, break them into multiple classes
        E.g. a class to get/set items from/to a cache should separate the behavior to connect to the cache server into another class
      • Check if function name has "and" or implies multiple operations. If so, consider breaking into multiple functions. However, keep the combined one if it's used by most clients:
        E.g. print_formatted function should be broken into two: print and formatted
      • Check if the API client has to copy-and-paste code to change some behavior. Provide hooks, methods to be overridden, instead.
      • Check if an attribute used internally in class behavior could be a get_something method
        E.g. on Django REST Framework, the CursorPagination class supported only a fixed page_size, because it didn't have the get_page_size method. This was changed by introducing the method.
      • Avoid hiding parameters that could be useful
        E.g. the API is calling another lower-level one, but it isn't exposing some useful parameters the lower-level API supports
      • Return everything the API client might need
      • Pass everything the API client might need when calling it's code (when Inversion of Control is in place)
      • Evaluate every mock.patch in the API tests. This indicates something isn't flexible to be changed at runtime. Consider having parameters to change those things
        E.g. Python's built-in sched.scheduler accepts timefunc and delayfunc, so the tests don't need to mock time.monotonic nor time.sleep and the clients can change those behaviors
  • Create abstractions
      • Wrap resources and functionalities common to multiple low-level implementations
        E.g. Beautiful Soup provides the same API for multiple parsers
      • Provide multiple abstraction levels, from the most simple to the most customizable
        E.g. Celery supports both the @app.task decorator and a custom task class that inherits from celery.Task
      • Provide the ability to get out of the abstraction cage and use directly the abstracted resources and functionalities
        E.g. Django querysets support .extra to combine custom SQL with ORM-generated one. It also supports .raw, which allow completely raw SQL queries
      • Wrap errors that are common to multiple low-level implementations and avoid leaking them directly to the API client
        E.g. a database connection error should be the same if your API supports multiple database engines. This helps API clients to change engines without changing much code
  • Be Pythonic
      • Use properties for simple getters and setters
      • Use magic methods for operator overloading
      • Use __repr__ magic method for simpler debug
      • Use with-statement contexts for open-close/start-end lifecycle
      • Use decorators to easily compose common behaviors or register something
      • Use iterators for lazy iterables
      • Use generators to intertwine logic
      • Use asyncio for async code
      • Use built-in collections instead of custom repositories
      • Accept simple high-order functions for simple Inversion of Control
        E.g. list.sort accepts key as the ranker function to calculate the order
  • Internationalize (I18N) the strings the end-user will see


  • Check for warning words like "warning", "careful", "remember to", "don't forget" in docs while describing a behavior. If they are there, consider changing behavior to be safer
  • Detect common mistakes and use Python's built-in warnings module to log warnings about them
  • Require unsafe behavior to be explicit
    • Don't show all if something isn't set, e.g., fields = None shouldn't mean all fields
  • Don't connect code implicitly by name or module. Use a registry or a registry-decorator
    • E.g. django-admin registry, which supports both register by function or by decorator
  • Avoid depending on order of method calls, try to use with-statement contexts instead
  • Fail-fast: crap in crap out is not a Pythonic idea
      • Raise ValueError if the library receives an invalid argument, like something that overflows, has a wrong format or is in a wrong state
         * - [ ] Raise TypeError if the library receives an argument with incompatible type, like a duck that doesn't quack. But don't do it inside a if isinstance(duck, LibDuck) or if type(duck) == LibDuck)! First try to call quack, then raise TypeError if it fails to give a clearer error


  • My API tries to make the simple easy, the complex possible and the wrong impossible