grappa is an declarative, verbose, and expressive assertion library for Python.
Pro Easy-to-understand syntax
Unlike other assertion libraries, grappa’s syntax is extremely verbose and expressive. It utilises English-like expressions and uses the OR operator (
|) for assertion. The assertion methods begin with either
expect. This makes grappa highly readable and easy to understand, even for non-developers.
Examples: to assert that a boolean is true:
True | should.be.true, and to assert that a value is within a range:
3.14 | should.be.within(2, 4).
Pro Ability to chain assertions with > operator
For complex assertions with multiple expressions, grappa allows you to chain them together with the
> operator. For example, to assert that “hello” should be a string and should be equal to “hello”, we can do:
'hello' | should.be.a(str) > should.be.equal.to('hello').
Pro Detailed error reporting
When an assertion fails in grappa, it outputs a detailed error report. The report contains the reason the assertion failed, the expected and received value(s) in plain English, e.g. "What we expected: an object that can be length measured and its length is equal to 4". It also contains the function grappa used to test the assertion (e.g.
len(), for our previous example error message) and provides additional information about the function, along with a link to the Python docs.
Con Can be too verbose for some people
Some consider grappa to be too verbose with unnecessary syntactic sugar. Compared to other assertion libraries, grappa is the most expressive of them all, which makes it a double-edged sword: the expressiveness makes it easy to read and understand, but each expression is longer to type and can seem redundant — the same assertion can be done with less code without grappa.