features¶

the basedpython language reference — one page per feature, each with the surface syntax, what it checks, and the python it lowers to

new here?

getting started installs by and walks a .by file through to running python. this page is the reference you come back to

python compatibility¶

.by is not a superset of .py

what the transpiled python does at runtime, beyond what you wrote

features that apply to a project rather than to a file

api lockfile (api.lock)
declared dependencies — imports checked against pyproject.toml, and a quick fix that declares them
editor support — postfix templates, completions, inlay hints and the outline
language injection — a string that says what language is written inside it
linting — the BY rules, and how ruff's own rules read .by source
the project server — by check answered out of the language server's warm state

what basedpython's vendored typeshed says that upstream's does not

typeshed improvements — covariant mapping keys, honest re groups, precise functools.cache, and the rest

type-checking improvements with no new syntax — they work in .by and .py files alike

fluid specializations
sound types — infer precise types instead of gradual ones
precise unsolved type variables — an unsolved type variable is Never, not Unknown
regex group types — type a match from the pattern it came from
boolean conditions — catch a test that conflates two members, or asks nothing
refutable unpacking — catch a, b = ... over a value of unknown length
unused return value — catch a call whose answer is thrown away
string formatting — check the format spec, and the value that has no rendering of its own

what a type is allowed to say

type parameters — their bounds, their variance, and what survives to runtime

the forms a class, function or binding can take

syntax inside a function body

how the formatter lays basedpython out