A modified version of excellent original checklist by Paul Wolf


  • Code is blackened with black
  • ruff has been run with no errors
  • mypy has been run with no errors
  • Function complexity problems have been resolved using the default complexity index of flake8.
  • Important core code can be loaded in iPython, ipdb easily.
  • There is no dead code
  • Comprehensions or generator expressions are used in place of for loops where appropriate
  • Comprehensions and generator expressions produce state but they do not have side effects within the expression.
  • Use zip(), any(), all(), filter(), etc. instead of for loops where appropriate
  • Functions that take as parameters and mutate mutable variables don’t return these variables. They return None.
  • Return immutable copies of mutable types instead of mutating the instances themselves when mutable types are passed as parameters with the intention of returning a mutated version of that variable.
  • Avoid method cascading on objects with methods that return self.
  • Function and method parameters never use empty collection or sequence instances like list [] or dict {}. Instead they must use None to indicate missing input
  • Variables in a function body are initialised with empty sequences or collections by callables, list(), dict(), instead of [], {}, etc.
  • Always use the Final type hint for class instance parameters that will not change.
  • Context-dependent variables are not unnecessarily passed between functions or methods
  • View functions either implement the business rules the view is repsonsible for or it passes data downstream to have this done by services and receives non-context dependent data back.
  • View functions don’t pass request to called functions
  • Functions including class methods don’t have too many local parameters or instance variables. Especially a class’ __init__() should not have too many parameters.
  • Profiling code is minimal
  • Logging is the minimum required for production use
  • There are no home-brewed solutions for things that already exist in the PSL (python standard library)

n00b habbits

  • Bare except clause, Python uses exceptions to flag system level interupts such as sigkills. Don’t do this.
  • Argument default mutatable arguments such as def foo(bar=[]) are defined when the function is defined, not when its run, and will result in a all function calls sharing the same instance of bar
  • Checking for equality using ==. Due to inheritance this is not desirable as it pins to a concrete type and not potentially it descendents. In other words the Liskov substitution principle. Instead isinstance(p, tuple)
  • Explicit bool or length checks, such as if bool(x) or if len(x) > 0 is redundant, as Python has sane truthy evaluation.
  • Use of range over the for in idiom
  • If you really need the index, always use enumerate
  • Not using items() on a dict for k, v in dict.items())
  • Using time.time to measure code performance. Use time.perf_counter instead.
  • Using print statements over logging
  • Using import * will normally liter the namespace with variable. Dont be lazy, be specific. from itertools import count

Imports and modules

  • Imports are sorted by isort or according to some standard that is consistent within the team
  • Import packages or modules to qualify the use of functions or classes so that unqualified function calls can be assumed to be to functions in the current module


  • Modules have docstrings
  • Classes have docstrings unless their purpose is immediately obvious
  • Methods and functions have docstrings
  • Comments and docstrings add non-obvious and helpful information that is not already present in the naming of functions and variables

General Complexity

  • Functions as complex as they need to be but no more (as defined by flake8\ ’s default complexity threshold)
  • Classes have only as many methods as required and have a simple hierarchy

Context Freedom

  • All important functionality can be loaded easily in ipython without having to construct dummy requests, etc.
  • All important functionality can be loaded in pdb (or a variant, ipdb, etc.)


  • Use immutable types ()tuple, frozenset, Enum, etc) over mutable types whenever possible


  • Functions are pure wherever possible, i.e. they take input and provide a return value with no side-effects or reliance on hidden state.


  • Module level variables do not take context-dependent values like connection clients to remote systems unless the client is used immediately for another module level variable and not used again


  • Every class has a single well-defined purpose. That is, the class does not mix up different tasks, like remote state acquisition, web sockets notification, data formatting, etc.
  • Classes manage state and do not just represent the encapsulation of behaviour
  • All methods access either cls or self in the body. If a method does not access cls or self, it should be a function at module level.
  • @classmethod is used in preference to @staticmethod but only if the method body accesses cls otherwise the method should be a module level function.
  • Constants are declared at module level not in methods or class level
  • Constants are always upper case
  • Abstract classes are derived from abc: from abc import ABC
  • Abstract methods use the @abstractmethod decorator
  • Abstract class properties use both @abstractmethod and @property decorators
  • Classes do not use multiple inheritance
  • Classes do not use mixins (use composition instead) except in rare cases
  • Class names do not use the word “Base” to signal they are the single ancestor, like “BaseWhatever”
  • Decorators are not used to replace classes as a design pattern
  • __init__() does not define too many local variables. Use the Parameter Consolidation pattern instead.
  • A factory class or function at module level is used for complex class construction (see Design Patterns) to achieve composition
  • Classes are not dynamically created from strings except where forward reference requires this

Design Patterns

  • Do not use designs that cause a typical Python developer to have to learn new semantics that are unexpected in Python
  • Classes primarily use composition in preference to inheritance
  • Beyond a very small number of simple variables, a class’ purpose is to acquire state for another class or it uses another class to acquire state in particular if the state is from a remote service.
  • If you use the Context Parameter pattern, it is critical that the state of the context does not change after calling its __init__(), i.e. it should be immutable
  • If a class’ purpose is to represent an external integration, you probably want numerous classes to compose the service: RemoteDataClient, DomainManager, ContextManager, Factory, NotificationController, DomainResponse, DataFormatter and so on.