按需加载工作表 ¶

This feature, new in version 0.7.1, is governed by the on_demand argument to the 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):
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:: open_workbook() loads global data and returns without releasing resources. At this stage, the only information available about sheets is Book.nsheets and Book.sheet_names() .

Book.sheet_by_name() and Book.sheet_by_index() will load the requested sheet if it is not already loaded.

Book.sheets() will load all unloaded sheets.

The caller may save memory by calling 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 Book.sheet_by_name() or 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 Book.sheet_loaded() .

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 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 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 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.