pypi i Expression


Pragmatic functional programming for Python inspired by F#

by cognitedata

2.0.0 (see all)License:MIT LicenseCategories:Vanilla Python Functional Programming
pypi i Expression


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Pragmatic functional programming

Expression aims to be a solid, type-safe, pragmatic, and high performance library for frictionless and practical functional programming in Python 3.9+.

By pragmatic we mean that the goal of the library is to use simple abstractions to enable you to do practical and productive functional programming in Python (instead of being a Monad tutorial).

Python is a multi-paradigm programming language that also supports functional programming constructs such as functions, higher-order functions, lambdas, and in many ways favors composition over inheritance.

Better Python with F#

Expression tries to make a better Python by providing several functional features inspired by F# into Python. This serves several purposes:

  • Enable functional programming in a Pythonic way. I.e make sure we are not over-abstracting things. Expressions will not be anywhere close to e.g Haskell.
  • Everything you learn with Expression can also be used with F#. Learn F# by starting in a programming language they already know. Perhaps get inspired to also try out F# by itself.
  • Make it easier for F# developers to use Python when needed, and re-use many of the concepts and abstractions they already know and love.

Expression will enable you to work with Python using many of the same programming concepts and abstractions. This enables concepts such as Railway oriented programming (ROP) for better and predictable error handling. Pipelining for workflows, computational expressions, etc.

Expressions evaluate to a value. Statements do something.

F# is a functional programming language for .NET that is succinct (concise, readable, and type-safe) and kind of Pythonic. F# is in many ways very similar to Python, but F# can also do a lot of things better than Python:

  • Strongly typed, if it compiles it usually works making refactoring much safer. You can trust the type-system. With mypy or Pylance you often wonder who is right and who is wrong.
  • Type inference, the compiler deduces types during compilation
  • Expression based language

Getting Started

You can install the latest expression from PyPI by running pip (or pip3). Note that expression only works for Python 3.9+.

$ pip3 install expression


Functional Programming in Python:

  1. Introduction
  2. Collections
  3. Lambda Calculus
  4. Optional Values
  5. Railway Oriented Programming
  6. Effects and Side-effects


  • Industrial strength library for functional programming in Python.
  • The resulting code should look and feel like Python (PEP-8). We want to make a better Python, not some obscure DSL or academic Monad tutorial.
  • Provide pipelining and pipe friendly methods. Compose all the things!
  • Dot-chaining on objects as an alternative syntax to pipes.
  • Lower the cognitive load on the programmer by:
    • Avoid currying, not supported in Python by default and not a well known concept by Python programmers.
    • Avoid operator (|, >>, etc) overloading, this usually confuses more than it helps.
    • Avoid recursion. Recursion is not normally used in Python and any use of it should be hidden within the SDK.
  • Provide type-hints for all functions and methods.
  • Code must pass strict static type checking by pylance. Pylance is awesome, use it!

Supported features

Expression will never provide you with all the features of F# and .NET. We are providing a few of the features we think are useful, and will add more on-demand as we go along.

  • Pipelining - for creating workflows.
  • Composition - for composing and creating new operators
  • Fluent or Functional syntax, i.e dot chain or pipeline operators.
  • Pattern Matching - an alternative flow control to if-elif-else.
  • Error Handling - Several error handling types
    • Option - for optional stuff and better None handling.
    • Result - for better error handling and enables railway-oriented programming in Python.
    • Try - a simpler result type that pins the error to an Exception.
  • Collections - immutable collections.
    • Sequence - a better itertools and fully compatible with Python iterables.
    • FrozenList - a frozen and immutable list type.
    • Map - a frozen and immutable dictionary type.
    • AsyncSeq - Asynchronous iterables.
    • AsyncObservable - Asynchronous observables. Provided separately by aioreactive.
  • Effects: - lightweight computational expressions for Python. This is amazing stuff.
    • option - an optional world for working with optional values.
    • result - an error handling world for working with result values.
  • Mailbox Processor: for lock free programming using the Actor model.
  • Cancellation Token: for cancellation of asynchronous (and synchronous) workflows.
  • Disposable: For resource management.


Expression provides a pipe function similar to |> in F#. We don't want to overload any Python operators e.g | so pipe is a plain old function taking N-arguments, and will let you pipe a value through any number of functions.

from expression import pipe

gn = lambda g: g * y
fn = lambda x: x + z
value = pipe(

assert value == gn(fn(x))

Expression objects also have a pipe method so you can dot chain pipelines directly on the object:

from expression import pipe

gn = lambda g: g * y
fn = lambda x: x + z
value = x.pipe(

assert value == gn(fn(x))

So for example with sequences you may create sequence transforming pipelines:

from expression.collections import seq

ys = xs.pipe( x: x * 10),
    seq.filter(lambda x: x > 100),
    seq.fold(lambda s, x: s + x, 0)


Functions may even be composed directly into custom operators:

from expression import compose
from expression.collections import seq

custom = compose( x: x * 10),
    seq.filter(lambda x: x > 100),
    seq.fold(lambda s, x: s + x, 0)

ys = custom(xs)

Fluent and Functional

Expression can be used both with a fluent or functional syntax (or both.)

Fluent syntax

The fluent syntax uses methods and is very compact. But it might get you into trouble for large pipelines since it's not a natural way of adding line breaks.

xs = Seq.of(1, 2, 3)
ys = x: x * 100).filter(lambda x: x > 100).fold(lambda s, x: s + x, 0)

Note that fluent syntax is probably the better choice if you use mypy for type checking since mypy may have problems inferring types through larger pipelines.

Functional syntax

The functional syntax is a bit more verbose but you can easily add new operations on new lines. The functional syntax is great to use together with pylance/pyright.

from expression import pipe
from expression.collections import seq, Seq

xs = Seq.of(1, 2, 3)
ys = pipe(xs, x: x * 100),
    seq.filter(lambda x: x > 100),
    seq.fold(lambda s, x: s + x, 0),

Both fluent and functional syntax may be mixed and even pipe can be used fluently.

xs = Seq.of(1, 2, 3).pipe(


The option type is used when a function or method cannot produce a meaningful output for a given input.

An option value may have a value of a given type i.e Some(value), or it might not have any meaningful value, i.e Nothing.

from expression import Some, Nothing, Option

def keep_positive(a: int) -> Option[int]:
    if a > 0:
        return Some(a)

    return Nothing
def exists(x : Option[int]) -> bool:
    for value in x.match(Ok):
        return True

    return False

Options as effects

Effects in Expression is implemented as specially decorated coroutines (enhanced generators) using yield, yield from and return to consume or generate optional values:

from expression import effect, Some

def fn():
    x = yield 42
    y = yield from Some(43)

    return x + y

xs = fn()

This enables "railway oriented programming" e.g if one part of the function yields from Nothing then the function is side-tracked (short-circuit) and the following statements will never be executed. The end result of the expression will be Nothing. Thus results from such an option decorated function can either be Ok(value) or Error(error_value).

from expression import effect, Some, Nothing

def fn():
    x = yield from Nothing # or a function returning Nothing

    # -- The rest of the function will never be executed --
    y = yield from Some(43)

    return x + y

xs = fn()
assert xs is Nothing

For more information about options:


The Result[T, TError] type lets you write error-tolerant code that can be composed. A Result works similar to Option but lets you define the value used for errors, e.g an exception type or similar. This is great when you want to know why some operation failed (not just Nothing).

from expression import effect, Result, Ok, Error, pipe

@effect.result[int, Exception]()
def fn():
    x = yield from Ok(42)
    y = yield from OK(10)
    return x + y

xs = fn()
assert isinstance(xs, Some)

A simplified type called Try is also available. It's a result type that is pinned to Exception i.e Result[TSource, Exception].


Contains operations for working with iterables so all the functions in the sequence module will work with Python iterables. Iterables are immutable by design, and perfectly suited for functional programming.

# Normal python way. Nested functions are hard to read since you need to
# start reading from the end of the expression.
xs = range(100)
ys = functools.reduce(lambda s, x: s + x, filter(lambda x: x > 100, map(lambda x: x * 10, xs)), 0)

# With Expression you pipe the result so it flows from one operator to the next:
ys = pipe(
    xs, x: x * 10),
    seq.filter(lambda x: x > 100),
    seq.fold(lambda s, x: s + x, 0),
assert ys == zs

Pattern Matching

Pattern matching is tricky for a language like Python. We are waiting for PEP 634 and structural pattern matching for Python. But we need something that can by handled by static type checkers and will also decompose or unwrap inner values.

What we want to achieve with pattern matching:

  • Check multiple cases with default handling if no match is found.
  • Only one case will ever match. This reduces the cognitive load on the programmer.
  • Type safety. We need the code to pass static type checkers.
  • Decomposing of wrapped values, e.g options, and results.
  • Case handling must be inline, i.e we want to avoid lambdas which would make things difficult for e.g async code.
  • Pythonic. Is it possible to use something that still looks like Python code?

The solution we propose is based on loops, singleton iterables and resource management. This lets us write our code inline, decompose, and unwrap inner values, and also effectively skip the cases that do not match.

from expression import match

with match("expression") as case:
    if case("rxpy"):  # will not match
        assert False

    for value in case(str):  # will match
        assert value == "expression"

    for value in case(float):  # will not match
        assert False

    if case._:  # will run if any previous case does not match
        assert False

Using match as a context manager will make sure that a case was actually found. You might need to have a default handler to avoid MatchFailureError.

Test cases may be additionally be wrapped in a function to have a match expression that returns a value:

def matcher(value) -> Option[int]:
    with match(value) as case:
        for value in case(Some[int]):
            return Some(42)

        if case._:
            return Some(2)

    return Nothing

result = matcher(42).

Classes may also support match fluently, i.e: xs.match(pattern). If you add generic types to the pattern then unwrapped values will get the right type without having to cast.

    xs = Some(42)
    ys = x: x + 1)

    for value in ys.match(Some[int]):
        assert value == 43
        assert False

Pattern matching can also be used with destructuring of e.g iterables:

xs: FrozenList[int] = empty.cons(42)
for (head, *tail) in xs.match(FrozenList):
    assert head == 42

Classes can support more advanced pattern matching and decompose inner values by subclassing or implementing the matching protocol.

class SupportsMatch(Protocol[TSource]):
    """Pattern matching protocol."""

    def __match__(self, pattern: Any) -> Iterable[TSource]:
        """Return a singleton iterable item (e.g `[value]`) if pattern
        matches, else an empty iterable (e.g. `[]`)."""
        raise NotImplementedError

This significantly simplifies the decomposition and type handling compared to using isinstance directly. E.g code from aioreactive:

if isinstance(msg, InnerObservableMsg):
    msg = cast(InnerObservableMsg[TSource], msg)
    xs: AsyncObservable[TSource] = msg.inner_observable

Now becomes:

with match(msg) as case:
    for xs in case(InnerObservableMsg[TSource]):

Note that the matching protocol may be implemented by both values and patterns. Patterns implementing the matching protocol effectively becomes active patterns.

class ParseInteger_(SupportsMatch[int]):
    """Active pattern for parsing integers."""

    def __match__(self, pattern: Any) -> Iterable[int]:
        """Match value with pattern."""

            number = int(pattern)
        except ValueError:
            return []
            return [number]

ParseInteger = ParseInteger_()  # Pattern singleton instance

text = "42"
with match(text) as case:
    for value in case(ParseInteger):
        assert value == int(text)

    if case._:
        assert False

Notable Differences

In F# modules are capitalized, in Python they are lowercase (PEP-8). E.g in F# Option is both a module (OptionModule internally) and a type. In Python the module is option and the type is capitalized i.e Option.

Thus in Expression you use option as the module to access module functions such as and the name Option for the type itself.

>>> from expression import Option, option
>>> Option
<class 'expression.core.option.Option'>
>>> option
<module 'expression.core.option' from '/Users/dbrattli/Developer/Github/Expression/expression/core/'>

F# pattern matching is awesome and the alternative we present here cannot be compared. But it helps us match and decompose without having to type-cast every time.

Common Gotchas and Pitfalls

A list of common problems and how you may solve it:

Expression is missing the function/operator I need

Remember that everything is a function, so you can easily implement the function yourself and use it with Expression. If you think the function is also usable for others, then please open a PR to include it with Expression.

Resources and References

A collection of resources that were used as reference and inspiration for creating this library.

How-to Contribute

You are very welcome to contribute with suggestions or PRs 😍 It is nice if you can try to align the code and naming with F# modules, functions, and documentation if possible. But submit a PR even if you should feel unsure.

Code, doc-strings, and comments should also follow the Google Python Style Guide.

Code checks are done using

To run code checks on changed files every time you commit, install the pre-commit hooks by running:

pre-commit install

Code of Conduct

This project follows, see our Code of Conduct.



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