A plugin to fake subprocess for pytest
Project description
pytest-subprocess
A plugin to fake subprocess for pytest
Usage
The plugin adds the fake_subprocess fixture. It can be used it to register subprocess results so you won’t need to rely on the real processes. The plugin hooks on the subprocess.Popen(), which is the base for other subprocess functions. That makes the subprocess.run(), subprocess.call(), subprocess.check_call() and subprocess.check_output() methods also functional.
Installation
You can install pytest-subprocess via pip from PyPI:
$ pip install pytest-subprocess
Basic usage
The most important method is fake_process.register_subprocess() which allows defining the fake processes behavior.
def test_echo_null_byte(fake_process):
fake_process.register_subprocess(
["echo", "-ne", "\x00"], stdout=bytes.fromhex("00")
)
process = subprocess.Popen(
["echo", "-ne", "\x00"], stdout=subprocess.PIPE,
)
out, _ = process.communicate()
assert process.returncode == 0
assert out == b"\x00"Optionally, the stdout and stderr parameters can be a list (or tuple) of lines to be joined together with a trailing os.linesep on each line.
def test_git(fake_process):
fake_process.register_subprocess(
["git", "branch"], stdout=["* fake_branch", " master"]
)
process = subprocess.Popen(
["git", "branch"],
stdout=subprocess.PIPE,
universal_newlines=True,
)
out, _ = process.communicate()
assert process.returncode == 0
assert out == "* fake_branch\n master\n"Passing input
By default, if you use input argument to the Popen.communicate() method, it won’t crash, but also won’t do anything useful. By passing a function as stdin_callable argument for the fake_process.register_subprocess() method you can specify the behavior based on the input. The function shall accept one argument, which will be the input data. If the function will return a dictionary with stdout or stderr keys, its value will be appended to according stream.
def test_pass_input(fake_process):
def stdin_function(input):
return {
"stdout": "This input was added: {data}".format(
data=input.decode()
)
}
fake_process.register_subprocess(
["command"],
stdout=[b"Just stdout"],
stdin_callable=stdin_function,
)
process = subprocess.Popen(
["command"], stdin=subprocess.PIPE, stdout=subprocess.PIPE,
)
out, _ = process.communicate(input=b"sample input\n")
assert out.splitlines() == [
b"Just stdout",
b"This input was added: sample input",
]Unregistered commands
By default, when the fake_process fixture is being used, any attempt to run subprocess that has not been registered will raise the ProcessNotRegisteredError exception. To allow it, use fake_process.allow_unregistered(True), which will execute all unregistered processes with real subprocess, or use fake_process.pass_command("command") to allow just a single command.
def test_real_process(fake_process):
with pytest.raises(pytest_subprocess.ProcessNotRegisteredError):
# this will fail, as "ls" command is not registered
subprocess.call("ls")
fake_process.pass_command("ls")
# now it should be fine
assert subprocess.call("ls") == 0
# allow all commands to be called by real subprocess
fake_process.allow_unregistered(True)
assert subprocess.call(["ls", "-l"]) == 0Differing results
Each register_subprocess() or pass_command() method call will register only one command execution. You can call those methods multiple times, to change the faked output on each subprocess run. When you call subprocess more will be raised. To prevent that, call fake_process.keep_last_process(True), which will keep the last registered process forever.
def test_different_output(fake_process):
# register process with output changing each execution
fake_process.register_subprocess("test", stdout="first execution")
# the second execution will return non-zero exit code
fake_process.register_subprocess(
"test", stdout="second execution", returncode=1
)
assert subprocess.check_output("test") == b"first execution"
second_process = subprocess.run("test", stdout=subprocess.PIPE)
assert second_process.stdout == b"second execution"
assert second_process.returncode == 1
# 3rd time shall raise an exception
with pytest.raises(pytest_subprocess.ProcessNotRegisteredError):
subprocess.check_call("test")
# now, register two processes once again,
# but the last one will be kept forever
fake_process.register_subprocess("test", stdout="first execution")
fake_process.register_subprocess("test", stdout="second execution")
fake_process.keep_last_process(True)
# now the processes can be called forever
assert subprocess.check_output("test") == b"first execution"
assert subprocess.check_output("test") == b"second execution"
assert subprocess.check_output("test") == b"second execution"
assert subprocess.check_output("test") == b"second execution"Using callbacks
You can pass a function as callback argument to the register_subprocess() method which will be executed instead of the real subprocess. The callback function can raise exceptions which will be interpreted in tests as an exception raised by the subprocess. The fixture will pass FakePopen class instance into the callback function, that can be used to change the return code or modify output streams.
def callback_function(process):
process.returncode = 1
raise PermissionError("exception raised by subprocess")
def test_raise_exception(fake_process):
fake_process.register_subprocess(["test"], callback=callback_function)
with pytest.raises(
PermissionError, match="exception raised by subprocess"
):
process = subprocess.Popen(["test"])
process.wait()
assert process.returncode == 1It is possible to pass additional keyword arguments into callback by using the callback_kwargs argument:
def callback_function_with_kwargs(process, return_code):
process.returncode = return_code
def test_callback_with_arguments(fake_process):
return_code = 127
fake_process.register_subprocess(
["test"],
callback=callback_function_with_kwargs,
callback_kwargs={"return_code": return_code},
)
process = subprocess.Popen(["test"])
process.wait()
assert process.returncode == return_codeAs a context manager
The fake_process fixture provides context() method that allows us to use it as a context manager. It can be used to limit the scope when a certain command is allowed, e.g. to make sure that the code doesn’t want to execute it somewhere else.
def test_context_manager(fake_process):
with pytest.raises(pytest_subprocess.ProcessNotRegisteredError):
# command not registered, so will raise an exception
subprocess.check_call("test")
with fake_process.context() as nested_process:
nested_process.register_subprocess("test", occurrences=3)
# now, we can call the command 3 times without error
assert subprocess.check_call("test") == 0
assert subprocess.check_call("test") == 0
# the command was called 2 times, so one occurrence left, but since the
# context manager has been left, it is not registered anymore
with pytest.raises(pytest_subprocess.ProcessNotRegisteredError):
subprocess.check_call("test")Non-exact command matching
If you need to catch a command with some non-predictable elements, like a path to a randomly-generated file name, you can use fake_subprocess.any() for that purpose. The number of arguments that should be matched can be controlled by min and max arguments. To use fake_subprocess.any() you need to define the command as a tuple or list. The matching will work even if the subprocess command will be called with a string argument.
def test_non_exact_matching(fake_process):
# define a command that will take any number of arguments
fake_process.register_subprocess(["ls", fake_process.any()])
assert subprocess.check_call("ls -lah") == 0
# `fake_subprocess.any()` is OK even with no arguments
fake_process.register_subprocess(["ls", fake_process.any()])
assert subprocess.check_call("ls") == 0
# but it can force a minimum amount of arguments
fake_process.register_subprocess(["cp", fake_process.any(min=2)])
with pytest.raises(pytest_subprocess.ProcessNotRegisteredError):
# only one argument is used, so registered command won't match
subprocess.check_call("cp /source/dir")
# but two arguments will be fine
assert subprocess.check_call("cp /source/dir /tmp/random-dir") == 0
# the `max` argument can be used to limit maximum amount of arguments
fake_process.register_subprocess(["cd", fake_process.any(max=1)])
with pytest.raises(pytest_subprocess.ProcessNotRegisteredError):
# cd with two arguments won't match with max=1
subprocess.check_call("cd ~/ /tmp")
# but any single argument is fine
assert subprocess.check_call("cd ~/") == 0
# `min` and `max` can be used together
fake_process.register_subprocess(
["my_app", fake_process.any(min=1, max=2)]
)
assert subprocess.check_call(["my_app", "--help"]) == 0Check if process was called
You may want to simply check if a certain command was called, you can do this by accessing fake_process.calls, where all commands are stored as-called. You can also use a utility function fake_process.call_count() to see how many a command has been called. The latter supports fake_process.any().
def test_check_if_called(fake_process):
fake_process.keep_last_process(True)
# any command can be called
fake_process.register_subprocess([fake_process.any()])
subprocess.check_call(["cp", "/tmp/source", "/source"])
subprocess.check_call(["cp", "/source", "/destination"])
subprocess.check_call(["cp", "/source", "/other/destination"])
# you can check if command is in ``fake_process.calls``
assert ["cp", "/tmp/source", "/source"] in fake_process.calls
assert ["cp", "/source", "/destination"] in fake_process.calls
assert ["cp", "/source", "/other/destination"] in fake_process.calls
# or check how many it was called, possibly with wildcard arguments
assert fake_process.call_count(["cp", "/source", "/destination"]) == 1
# with ``call_count()`` you don't need to use the same type as
# the subprocess was called
assert fake_process.call_count("cp /tmp/source /source") == 1
# can be used with ``fake_process.any()`` to match more calls
assert fake_process.call_count(["cp", fake_process.any()]) == 3Handling signals
You can use standard kill(), terminate() or send_signal() methods in Popen instances. There is an additional received_signals() method to get a tuple of all signals received by the process. It is also possible to set up an optional callback function for signals.
import signal
def test_signal_callback(fake_process):
"""Test that signal callbacks work."""
def callback(process, sig):
if sig == signal.SIGTERM:
process.returncode = -1
fake_process.register_subprocess("test", signal_callback=callback)
process = subprocess.Popen("test")
process.send_signal(signal.SIGTERM)
process.wait()
assert process.returncode == -1
assert process.received_signals() == (signal.SIGTERM,)Asyncio support
The plugin now supports asyncio and works for asyncio.create_subprocess_shell and asyncio.create_subprocess_exec:
@pytest.mark.asyncio
async def test_basic_usage(fake_process,):
fake_process.register_subprocess(
["some-command-that-is-definitely-unavailable"], returncode=500
)
process = await asyncio.create_subprocess_shell(
"some-command-that-is-definitely-unavailable"
)
returncode = await process.wait()
assert process.returncode == returncode
assert process.returncode == 500Documentation
For full documentation, including API reference, please see https://pytest-subprocess.readthedocs.io/en/latest/.
Contributing
Contributions are very welcome. Tests can be run with tox, please ensure the coverage at least stays the same before you submit a pull request.
License
Distributed under the terms of the MIT license, “pytest-subprocess” is free and open source software
Issues
If you encounter any problems, please file an issue along with a detailed description.
This pytest plugin was generated with Cookiecutter along with @hackebrot’s cookiecutter-pytest-plugin template.
History
1.3.2 (2021-11-07)
Bug fixes
#61: Fixed behavior of asyncio.create_subproess_exec().
1.3.1 (2021-11-01)
Bug fixes
#58: Correctly handle file stream output.
1.3.0 (2021-10-24)
Features
#55: Add support for terminate(), kill(), send_signal().
1.2.0 (2021-10-09)
Features
Other changes
#50: Change docs theme.
1.1.2 (2021-07-17)
Bug fixes
#47: Prevent allow_unregistered() and keep_last_process() from affecting other tests.
1.1.1 (2021-06-18)
Bug fixes
#43: Wait for callback thread to finish when calling communicate().
Other changes
#42: Fix type annotations for register_subprocess().
1.1.0 (2021-04-18)
Bug fixes
#37: Preserve original command in proc.args to prevent leaking the internal Command type.
Other changes
1.0.1 (2021-03-20)
Bug fixes
#34: Prevent appending newlines to outputs unless defined as list/tuple.
Other changes
#32: Make the Command class iterable.
1.0.0 (2020-08-22)
Features
0.1.5 (2020-06-19)
Bug fixes
#26: encoding and errors arguments will properly trigger text mode.
0.1.4 (2020-04-28)
Bug fixes
#22: The returncode will not be ignored when callback is used.
#21: The exception raised from callback will take precedence over those from subprocess.
#20: Registering process will be now consistent regardless of the command type.
#19: Fixed crash for stderr redirect with an empty stream definition.
0.1.3 (2020-03-04)
Features
#13: Allow passing keyword arguments into callbacks.
Bug fixes
#12: Properly raise exceptions from callback functions.
Documentation changes
#15: Add documentation chapter about the callback functions.
0.1.2 (2020-01-17)
Features
#3: Add basic support for process input.
Bug fixes
#5: Make wait() method to raise TimeoutError after the desired time will elapse.
Documentation changes
Other changes
0.1.1 (2019-11-24)
Other changes
0.1.0 (2019-11-23)
Initial release
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