Starter Class¶
Your Starter will be used to customize how xprocess behaves. It must be a subclass of ProcessStarter where the required information to start a process instance will be provided.
Matching process output with pattern¶
In order to detect that your process is ready to answer queries, pytest-xprocess allows the user to provide a string pattern by setting the class variable pattern in the Starter class. pattern will be waited for in the process logfile for a maximum time defined by timeout before timing out in case the provided pattern is not matched.
It’s important to note that pattern is a regular expression and will be matched using python re.search, so usual regex syntax (e.g. "eggs\s+([a-zA-Z_][a-zA-Z_0-9]*") can be used freely.
@pytest.fixture
def myserver(xprocess):
class Starter(ProcessStarter):
# Here, we assume that our hypothetical process
# will print the message "server has started"
# once initialization is done
pattern = "[Ss]erver has started!"
# ...
Making sure your process is ready with startup_check¶
Some processes don’t have that much console output, so pytest-xprocess offers an alternative way to check if the initialized process is in a query-ready state by allowing the user to define a callback function startup_check.
When provided, this function will be called upon to check process responsiveness.
startup_check must return a boolean value (True or False)
@pytest.fixture
def myserver(xprocess):
class Starter(ProcessStarter):
# checks if our server is ready with a ping
def startup_check(self):
sock = socket.socket()
sock.connect(("myhostname", 6777))
sock.sendall(b"ping\n")
return sock.recv(1) == "pong!"
# ...
A note on pattern vs startup_check for detecting process initialization¶
Both pattern and startup_check are optional, users may chose to use what suites their needs the most. However, at least one of them must be specified since XProcess.ensure needs a way to detect process initialization. Bellow we have a simple breakdown of possible setups configurations:
Only``pattern``. When only a
patternis provided, then, naturally, onlypatternwill be taken into account during process startupOnly
startup_check. Analogous to above, when onlystartup_checkis provided, onlystartup_checkwill be considered during process startupBoth
patternandstartup_check. When both have been specified, both will be used together. In other words, bothpatternneeds to be matched andstartup_checkmust succeed for the process to be considered query-ready.
Controlling Startup Wait Time with timeout¶
Some processes naturally take longer to start than others. By default, pytest-xprocess will wait for a maxium of 120 seconds for a given process to start before raising a TimeoutError. Changing this value may be useful, for example, when the user knows that a given process would never take longer than a known amount of time to start under normal circumstances, so if it does go over this known upper boundary, that means something is wrong and the waiting process must be interrupted. The maximum wait time can be controlled through the class variable timeout.
@pytest.fixture
def myserver(xprocess):
class Starter(ProcessStarter):
# will wait for 10 seconds before timing out
timeout = 10
# ...
Passing command line arguments to your process with args¶
In order to start a process, pytest-xprocess must be given a command to be passed into the subprocess.Popen constructor. Any arguments passed to the process command can also be passed using args. As an example, if I usually use the following command to start a given process:
$> myproc -name "bacon" -cores 4 <destdir>
That would look like:
args = ['myproc', '-name', '"bacon"', '-cores', 4, '<destdir>']
when using args in pytest-xprocess to start the same process.
@pytest.fixture
def myserver(xprocess):
class Starter(ProcessStarter):
# will pass "$> myproc -name "bacon" -cores 4 <destdir>" to the
# subprocess.Popen constructor so the process can be started with
# the given arguments
args = ['myproc', '-name', '"bacon"', '-cores', 4, '<destdir>']
# ...
Customizing process initialization with popen_kwargs¶
A popen_kwargs variable may optionality be set in ProcessStarter. This variable can be used for passing keyword values to the subprocess.Popen constructor, giving the user more control over how the process is initialized.
@pytest.fixture
def myserver(xprocess):
class Starter(ProcessStarter):
# passing extra keyword values to
# sucprocess.Popen constructor
popen_kwargs = {
"shell": True,
"user": "my_username",
"universal_newlines": True,
}
# ...
Automatic clean-up with terminate_on_interrupt¶
ProcessStarter has an optional flag terminate_on_interrupt. This flag will
make xprocess attempt to terminate and clean up all started processes and their
resources upon interruptions during pytest runs (CTRL+C, SIGINT and internal
errors) if set to True. The flag will default to False.
@pytest.fixture
def myserver(xprocess):
class Starter(ProcessStarter):
# xprocess will now attempt to
# clean up for you upon interruptions
terminate_on_interrupt = True
# ...
Limiting number of lines searched for pattern with max_read_lines¶
If the specified string pattern can be found within the first n outputted lines, there’s no reason to search all the remaining output (possibly hundreds of lines or more depending on the process). For that reason, pytest-xprocess allows the user to limit the maxium number of lines outputted by the process that will be searched for the given pattern with max_read_lines.
If max_read_lines lines have been searched and pattern has not been found, a RuntimeError will be raised to let the user know that startup has failed.
When not specified, max_read_lines will default to 50 lines.
@pytest.fixture
def myserver(xprocess):
class Starter(ProcessStarter):
# search the first 12 lines for pattern, if not found
# a RuntimeError will be raised informing the user
max_read_lines = 12
# ...
Customizing process execution environment with env¶
By default, the execution environment of the main test process will be inherited by the invoked process. But, if desired, it’s possible to customize the environment in which the new process will be invoked by providing a mapping containg the desired environment variables and their respective values with env.
@pytest.fixture
def myserver(xprocess):
class Starter(ProcessStarter):
# checks if our server is ready with a ping
env = {"PYTHONPATH": str(some_path), "PYTHONUNBUFFERED": "1"}
# ...
Overriding Wait Behavior¶
To override the wait behavior, override ProcessStarter.wait. See the
xprocess.ProcessStarter interface for more details. Note that the
plugin uses a subdirectory in .pytest_cache to persist the process ID
and logfile information.
An Important Note Regarding Stream Buffering¶
There have been reports of issues with test suites hanging when users attempt to start external python processes with xprocess.ensure method. The reason for this is that pytest-xprocess relies on matching predefined string patterns written to your environment standard output streams to detect when processes start and python’s sys.stdout/sys.stderr buffering ends up getting in the way of that.
A possible solution for this problem is making both streams unbuffered by passing the -u command-line option to your process start command or setting the PYTHONUNBUFFERED environment variable.