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A Shell-command faker for Python Unit Testing

Project description


A module that creates fake binaries from valid configuration ( a yaml file or a dict ) and launch them in the context of your Python script, without efforts.


pip install fakear

Quick Start

This is a mock of a fake ls command, that

  - args:
      - dexter
    return_code: 0
    output: Omelette du Fromage

On Python, you can now use Fakear to fake ls behaviour and act according to our mock.

>>> from subprocess import run
>>> from fakear import Fakear
>>> p = run(["ls", "dexter"])
ls: dexter: No such file or directory
>>> p.returncode
>>> with Fakear(cfg="fake_ls.yaml"):
...   fake_cmd = run(["ls", "dexter"])
Omelette du Fromage
>>> fake_cmd.returncode

You can use it as well in your unit test routine:

from fakear import Fakear
from subprocess import check_output

def test_faked_ls(self):
    with Fakear(cfg="fake_ls.yaml"):
        proc = check_output(["ls", "dexter"])

    assert proc.decode() == "Omelette du fromage\n"

How does it works ?

An instance handles a specific configuration file, with scenarios for one or multiple commands to fake.

By default, an instance is deactivated, meaning you can still configure it before the fake happens

When activated:

  • The instance creates the folder that should contains the fake binaries (faked_path)
  • It builds shell scripts corresponding to all the scenarios
  • It ensures every scripts are runnable
  • It sets in os.environ["PATH"] the faked_path at first place

So, as long as the instance is activated, you can run faked commands with subprocess module.

When deactivated, the instance removes the faked_path from PATH and deletes the folder from the filesystem


YAML Files

Every program mock should start with the program name as a key. Then you can describe multiple behaviours for a given set of arguments.

__command_name__ :
  # Default output with no arguments
  - return_code: -1
    output: This is a fake program, please give the correct arguments
  # output with arguments
  - args:
    - first_arg
    - sec_arg
    return_code: 0
    output: This is an example of fake command
  # output_file with arguments
  - args:
    - first_arg
    - sec_arg
    return_code: 0
    output_file: semver.txt
__command2_name__ :
  # Default behaviour of a program generated by Fakear

You can use those options to customise your fake program:

  • args (Optionna̦l) : a list of positionnal arguments that invoke this fake output
  • return_code : the return code when the program exits
  • output: The raw data to output when you invoke the program with these args
  • output_file: The path of a file containing the output to show

Notice that if you mention no args to your list in a subcommand, it overrides the default behaviour of your fake program.

Also, you have to set either output or output_file keys in the same subcommand. Otherwise, it should throw an error.


You can use Fakear in two ways:

  • as an instance:
>>> fakear = Fakear(cfg="path_of_a_valid.yml")
>>> fakear.enable()
>>> fakear.disable()

A Fakear instance can be manually enabled or disabled with the correct methods

  • as a Context Manager (recommended):
with Fakear(cfg="fake_ls.yml"):

With the context manager, enable et disable are handled automatically, and you can set the fake path at instanciation

Init variables

Fakear(cfg="/path/cfg.yaml", raw=cfg_data, path="/tmp/bin")
  • cfg: Path of the config file to use
  • raw: Python dict
  • path: Path for fake binaries

You can use either cfg or raw but not both of them, as it should overlap scenarios of the same command and create undefined behaviour.

Available Methods

  • enable() : Activate the instance
  • deactivate() : Deactivate the instance
  • set_path(path: str) : Sets a new path for faked commands. It must be absolute. You can't modify path when the instance is activated


Feel free to open an issue on this repository.

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