invar-runtime
Lightweight runtime contracts for Python projects using Invar.
Installation
pip install invar-runtime
Usage
from invar_runtime import pre, post, Contract, NonEmpty, Positive
# Use built-in contracts
@pre(NonEmpty)
def first(xs: list) -> int:
return xs[0]
# Create custom contracts
Even = Contract(lambda x: x % 2 == 0, "even")
@pre(Positive & Even)
def half(n: int) -> int:
return n // 2
# Compose contracts
@post(NonEmpty)
def get_items() -> list:
return [1, 2, 3]
Available Contracts
Collections
NonEmpty- Collection has at least one elementSorted- Elements are in sorted orderUnique- No duplicate elementsSortedNonEmpty- Both sorted and non-empty
Numbers
Positive- Greater than zeroNonNegative- Greater than or equal to zeroNegative- Less than zeroInRange(lo, hi)- Value in [lo, hi]Percentage- Value in [0, 100]
Strings
NonBlank- Non-empty and not just whitespace
List Elements
AllPositive- All elements > 0AllNonNegative- All elements >= 0NoNone- No None values
Decorators
@must_use(reason)- Mark return value as must-use@must_close- Mark class as requiring explicit cleanup@strategy(**params)- Specify Hypothesis strategies@skip_property_test(reason)- Skip property-based testing (reason required)
Skip Property Test Usage
The @skip_property_test decorator requires a reason explaining why the function cannot be property-tested. Guard warns if used without justification.
# ✅ Good - with reason
@skip_property_test("no_params: Zero-parameter function, no inputs to vary")
def get_version() -> str:
return "1.0.0"
# ❌ Bad - Guard warns about missing reason
@skip_property_test
def my_func(): ...
Valid reason categories:
no_params:- Function has no parameters to teststrategy_factory:- Returns Hypothesis strategy, not testable dataexternal_io:- Requires database/network/filesystemnon_deterministic:- Output depends on time/random state
Loop Invariants
from invar_runtime import invariant
while lo < hi:
invariant(0 <= lo <= hi <= len(arr), "bounds check")
mid = (lo + hi) // 2
...
Development Tools
For static analysis and verification tools, install invar-tools:
pip install invar-tools
# or use without installing:
uvx invar-tools guard
License
Apache-2.0
1 maintainer
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