Writing Facts

Facts are written as Python classes. They provide a command (as either a string or method) and a process function. The command is executed on the target host and the output passed (as a list of lines) to the process handler to generate fact data. Facts can output anything, normally a list or dict.

Importing & Using Facts

Facts are available in a global namespace as attributes on host objects: host.fact.X. Custom fact classes will automatically be added to the namespace when the module containing them is imported. Note that the class name is converted to snake case, so MyFact becomes my_fact, available as host.fact.my_fact.

Example: getting swap status

This fact returns a boolean indicating whether swap is enabled. For this fact the command is declared as a class attribute.

from  pyinfra.api import FactBase

class SwapEnabled(FactBase):
    '''
    Returns a boolean indicating whether swap is enabled.
    '''

    command = 'swapon --show'

    def process(self, output):
        return len(output) > 0  # we have one+ lines

This fact could then be used like so:

is_swap_enabled = host.fact.swap_enabled

Example: getting the list of files in a directory

This fact returns a list of files found in a given directory. For this fact the command is delcated as a class method, indicating the fact takes arguments.

from pyinfra.api import FactBase

class FindFiles(FactBase):
    '''
    Returns a list of files from a start point, recursively using find.
    '''

    def command(self, name):
        # Find files in the given location
        return 'find {0} -type f'.format(name)

    def process(self, output):
        return output  # return the list of lines (files) as-is

This fact could then be used like so:

list_of_files = host.fact.find_files('/some/path')

Example: getting any output from a command

This fact returns the raw output of any command. For this fact the command is delcated as a class method, indicating the fact takes arguments.

from pyinfra.api import FactBase

class RawCommandOutput(FactBase):
    '''
    Returns the raw output of a command.
    '''

    def command(self, command):
        return command

    def process(self, output):
        return '\n'.join(output)  # re-join and return the output lines

This fact could then be used like so:

command_output = host.fact.raw_command_output('execute my command')