on_demand.rst

Loading worksheets on demand

.. currentmodule:: excelrd.book

This feature, new in version 0.7.1, is governed by the on_demand argument to the :func:`~excelrd.open_workbook` function and allows saving memory and time by loading only those sheets that the caller is interested in, and releasing sheets when no longer required.

on_demand=False (default):

No change. :func:`~excelrd.open_workbook` loads global data and all sheets, releases resources no longer required (principally the :class:`str` or :class:`mmap.mmap` object containing the Workbook stream), and returns.

on_demand=True and BIFF version < 5.0:

A warning message is emitted, on_demand is recorded as False, and the old process is followed.

on_demand=True and BIFF version >= 5.0:

:func:`~excelrd.open_workbook` loads global data and returns without releasing resources. At this stage, the only information available about sheets is :attr:`Book.nsheets` and :meth:`Book.sheet_names`.

:meth:`Book.sheet_by_name` and :meth:`Book.sheet_by_index` will load the requested sheet if it is not already loaded.

:meth:`Book.sheets` will load all unloaded sheets.

The caller may save memory by calling :meth:`Book.unload_sheet` when finished with the sheet. This applies irrespective of the state of on_demand.

The caller may re-load an unloaded sheet by calling :meth:`Book.sheet_by_name` or :meth:`Book.sheet_by_index`, except if the required resources have been released (which will have happened automatically when on_demand is false). This is the only case where an exception will be raised.

The caller may query the state of a sheet using :meth:`Book.sheet_loaded`.

:meth:`Book.release_resources` may used to save memory and close any memory-mapped file before proceeding to examine already-loaded sheets. Once resources are released, no further sheets can be loaded.

When using on-demand, it is advisable to ensure that :meth:`Book.release_resources` is always called, even if an exception is raised in your own code; otherwise if the input file has been memory-mapped, the :class:`mmap.mmap` object will not be closed and you will not be able to access the physical file until your Python process terminates. This can be done by calling :meth:`Book.release_resources` explicitly in the finally part of a try/finally block.

The Book object is also a context manager, so you can wrap your code in a with statement that will make sure underlying resources are closed.