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Klara is a static analysis tools to automatic generate test case, based on SMT (z3) solver, with a powerful ast level inference system. Klara will take python file as input and generate corresponding test file in pytest format, that attempt to cover all return values. For example, following function in file

def triangle(x: int, y: int, z: int) -> str:
    if x == y == z:
        return "Equilateral triangle"
    elif x == y or y == z or x == z:
        return "Isosceles triangle"
        return "Scalene triangle"

will generate

import test
def test_triangle_0():
    assert test.triangle(0, 0, 0) == 'Equilateral triangle'
    assert test.triangle(0, 0, 1) == 'Isosceles triangle'
    assert test.triangle(2, 0, 1) == 'Scalene triangle'

See the Klara's documentation at

Note: Klara is still in early experimental stage, notable missing features are loop, comprehension, module import, exceptions and many more. See limitations for full list. It probably will not run on real world projects, so it's best to cherry-pick a few interesting functions to generate the corresponding test case.


Klara can be installed via pip tool by using:

pip install klara


We can invoke klara on any python source file, and it will generate a corresponding pytest test file.

$ cat
def foo(x: int, y: int, z: str):
    if x + y > 2:
        return x + y + 12
    elif x < y:
        return x + y
    elif (z + "me") == "some":
        return z + "thing"
        return x - y

$ klara
$ cat
import contract_test

def test_foo_0():
    assert, 3, \'\') == 15
    assert, 1, \'\') == 1
    assert, 0, \'so\') == \'sothing\'
    assert, 0, \'\') == 0

Consult the quick start manual for more examples and guidance. To use it as a static analysis library, go to Inference.

Why Klara?

Klara works on ast level and it doesn't execute user code in any way, which is a very important difference compared to similar tool like Crosshair and Pynguin that utilize concolic symbolic execution that required user code execution that might cause unwanted side effects. Klara work on ast level, combine with data flow analysis that utilize Control Flow Graph(CFG), Static Single Assignment(SSA), use-def chain, etc... to build a powerful python inference system that leverages Z3-solver for constraints solving and path feasibility check. Because of this, Klara is able to operate on both python2/3 source code with the help of typed_ast. To specify the source code is in python 2, pass in -py 2 argument. It's python 3 by default.

Klara can also be used as a static analysis tool, allow user to define custom rule to identify programming bugs, error or enforcing coding standard. With SMT solver support, analysis will be more accurate and greatly reduce false-positive case. For example

import klara
tree = klara.parse("""
    def foo(v1: int):
        if v1 > 4:
            if v1 < 3:
                z = 1
                z = 2
            z = 3
        s = z
with klara.MANAGER.initialize_z3_var_from_func(tree.body[0]):

Will print out:

[2, 3]

Because z = 1 is not possible due to v1 > 4 and v1 < 3 is unsatisfiable

The inference system architecture and api is largely inspired by Astroid, a static inference library used by Pylint.

Klara utilize the inference system to generate test case, in other words, it generate test case for all possible return values of the function, instead of generate test case for all control path of the function.

To illustrate the point, consider the function below, with divide by zero vulnerabilities at line 3

def foo(v1: int, v2: float):
    if v1 > 10000:
        s = v1 / 0  # unused statement
    if v1 > v2:
        s = v1
        s = v2
    return s

Klara will generate test inputs below

import contract_test
def test_foo_0():
    assert, -1.0) == 0
    assert, 0.0) == 0.0

It doesn't generate input v1 > 10000, so the test case would not be able to find out the exceptions. This is because the s at line 3 is unused in the return value.

If we modify the second if statement to elif, which we'll be able to return the [s]{.title-ref} at line 3, klara will generate test inputs that cover v1 > 10000 case.

This is an important distinction with other automatic test case generation available now, because by only generate test case for return values, we can generate a minimal test case, and it's easier to customize how do Klara cover the function.

For example, say we are composing a complex system

    def main(number: int, cm: int, dc: int, wn: int):
        mc = 0
        if wn > 2:
            if number > 2 and number > 2 or number > 2:
                if number > 0:
                    if wn > 2 or wn > 2:
                        mc = 2
                        mc = 5
                    mc = 100
            mc = 1
        nnn = number * cm
        if cm <= 4:
            num_incr = 4
            num_incr = cm
        n_num_incr = nnn / num_incr
        nnn_left = dc * num_incr * (n_num_incr / 2 + n_num_incr % 2)
        nnn_right = nnn - nnn_left
        is_flag = nnn_right
        if is_flag:
            cell = Component(nnn_right, options=[mc])
            cell = Component(nnn_right)
        return cell

It isn't immediately clear to us how many possible return values there are. But we can utilize Klara to generate inputs instantly, below is the generated test

import contract_test
def test_main_0():
    assert contract_test.main(2, 4, 1, 3) is not None
    assert contract_test.main(2, 4, -1, 6) is not None
    assert contract_test.main(2, 4, 1, 4) is not None
    assert contract_test.main(-2, 4, 3, 4) is not None
    assert contract_test.main(-1, -1, -1, 2) is not None
    assert contract_test.main(0, 0, 0, 3) is not None
    assert contract_test.main(0, 0, 0, 6) is not None
    assert contract_test.main(0, 0, 0, 4) is not None
    assert contract_test.main(-2, 0, 0, 4) is not None
    assert contract_test.main(0, 0, 0, 0) is not None

Above generated 10 total results, which is product of nnn_right which have 2 possibilities and mc which have 5 possibilities.

Suppose that 10 tests input is too much, and we have determine that the options argument to Component is redundant to test, we can use Klara's custom plugin to selectively determine which part to ignore in test generation. Go to customize coverage strategy for more information.

After we have setup the plugin, Klara will generate following test

import contract_test
def test_main_0():
    assert contract_test.main(1, 3, 0, 0) is not None
    assert contract_test.main(0, 0, 0, 0) is not None

Which is only 2 combinations of nnn_right

Because Klara can't dynamically execute the code, it will provide extension to specify how to infer specific ast node or user defined type to make Klara 'smarter'. It's described in extending, extending user type and customize coverage strategy.


We use Poetry to manage dependencies. After poetry is installed, run:

$ poetry shell
$ poetry install

To run the test case, do:

$ poetry run pytest test


  • The architecture of the inference system is largely inspired by Astroid.
  • Special thanks to Dr. Poh for guiding the early stages of the project.


This project is licensed under the terms of the GNU Lesser General Public License.