import2ledger code structure
============================

Concepts
--------

The main workflow of the program passes through three different types with different responsibilities.

Entry data
~~~~~~~~~~

Data for an output entry is kept and passed around in a dict with the following contents:

``date``
  A datetime.date object (if this is omitted, the ``default_date`` hook will fill in the default date from the user's configuration)

``payee``
  A string

``amount``
  A string or other object that can be safely converted to a decimal.Decimal

``currency``
  A string with a three-letter code, uppercase, identifying the transaction currency

It can optionally include additional keys for use as template variables.

Importers
~~~~~~~~~

At a high level, importers read a source file, and generate data for output entries.

Class method ``can_handle(source_file)``
  Returns true if the importer can generate entries from the given source file object, false otherwise.

``__init__(source_file)``
  Initializes an importer to generate entries from the given source file object.

``__iter__()``
  Returns a iterator of entry data dicts.

Class attribute ``TEMPLATE_KEY``
  A string with the full key to load the corresponding template from the user's configuration (e.g., ``'template patreon income'``).

Hooks
~~~~~

Hooks make arbitrary transformations to entry data dicts.  Every entry data dict generated by an importer is run through every hook before being output.

``__init__(config)``
  Initializes the hook with the user's configuration.

``run(entry_data)``
  This method makes the hook's transformations to the entry data dict, if any.  If this method clears the entry data dict, that entry will not be output.

Templates
~~~~~~~~~

Templates receive entry data dicts and format them into final output entries.

``__init__(template_str)``
  Initializes the template from a single string, as read from the user's configuration.

``render(payee, amount, currency, date=None, **template_vars)``
  Returns a string with the output entry, using the given entry data.  This is expected to be called as ``render(**entry_data)``; the arguments take the same types as entry data dicts.

Loading importers and hooks
---------------------------

Importers and hooks are both loaded and found dynamically when the program starts.  This makes it easy to extend the program: you just need to write the class following the established pattern, no registration needed.

import2ledger finds importers by looking at all ``.py`` files in the ``importers/`` directory, skipping files whose names start with ``.`` (hidden) or ``_`` (private).  It tries to import that file as a module.  If it succeeds, it looks for things in the module named ``*Importer``, and adds those to the list of importers.

Hooks follow the same pattern, searching the ``hooks/`` directory and looking for things named ``*Hook``.

Technically this is done by ``importers.load_all()`` and ``hooks.load_all()`` functions, but most of the code to do this is in the ``util`` module.

Main loop
---------

At a high level, import2ledger handles each input file this way::

  usable_importers = importers where can_handle(input_file) returns true
  for importer_class in usable_importers:
    template = built from importer_class.TEMPLATE_KEY
    input_file.seek(0)
    for entry_data in importer_class(input_file):
      for hook in all_hooks:
        hook.run(entry_data)
      if entry_data:
        template.render(**entry_data)

Note in particular that multiple importers can handle the same input file.  This helps support inputs like Patreon's earnings CSV, where completely different transactions are generated from the same source.