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Tools for falsifying hypothesis with random data generation

Project description

Hypothesis is a library for falsifying its namesake.

The primary entry point into the library is the hypothesis.falsify method.

What does it do?

You give it a predicate and a specification for how to generate arguments to that predicate and it gives you a counterexample.


In [1]: from hypothesis import falsify

In [2]: falsify(lambda x,y,z: (x + y) + z == x + (y +z), float,float,float)
Out[2]: (1.0, 2.0507190744664223, -10.940188909437985)

In [3]: falsify(lambda x: sum(x) < 100, [int])
Out[3]: ([6, 29, 65],)

In [4]: falsify(lambda x: sum(x) < 100, [int,float])
Out[4]: ([18.0, 82],)

In [12]: falsify(lambda x: "a" not in x, str)
Out[12]: ('a',)

Sometimes we ask it to falsify things that are true:

In [13]: falsify(lambda x: x + 1 == 1 + x, int)
Unfalsifiable: Unable to falsify hypothesis <function <lambda> at 0x2efb1b8>

of course sometimes we ask it to falsify things that are false but hard to find:

In [16]: falsify(lambda x: x != "I am the very model of a modern major general", str)
Unfalsifiable: Unable to falsify hypothesis <function <lambda> at 0x2efb398>

It’s not magic, and when the search space is large it won’t be able to do very much for hard to find examples.

How does it work?

Fundamentally it knows how to do two things with types:

  1. Generate them
  2. Minimize them

The API for generation is that you give it a generator specification and a size parameter and it generates values of “about that size”, for some completely unspecified interpretation of that meaning (each type is permitted to interpret it differently).

Mininimizing takes a value and returns an iterator over “minimized forms of that value”. Again for some completely unspecified and fuzzy meaning.

Falsification feeds various size parameters into the generation until it finds a counter example it likes, then minimizes that counter-example in a depth first manner to produce its end results.

WARNING: This software should be considered super pre alpha. It probably works pretty well, maybe, perhaps, but the API has had almost zero design gone into it and is likely to change radically once I actually start thinking about what it should look like.

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