% SPDX-FileCopyrightText: 2022 James R. Barlow % SPDX-License-Identifier: CC-BY-SA-4.0

Using the OCRmyPDF API

OCRmyPDF originated as a command line program and continues to have this legacy, but parts of it can be imported and used in other Python applications.

Some applications may want to consider running ocrmypdf from a subprocess call anyway, as this provides isolation of its activities.

Example

OCRmyPDF provides one high-level function to run its main engine from an application.

{versionchanged}

The {func}`ocrmypdf.ocr` function now accepts an {class}`~ocrmypdf.OcrOptions`
object as its first argument, providing a cleaner API with full type hints
and validation. The previous positional argument style remains supported.

Modern API (recommended)

The recommended way to call {func}ocrmypdf.ocr is to construct an {class}~ocrmypdf.OcrOptions object with all settings, then pass it as the sole argument:

python

import ocrmypdf
from ocrmypdf import OcrOptions

if __name__ == '__main__':  # To ensure correct behavior on Windows and macOS
    options = OcrOptions(
        input_file='input.pdf',
        output_file='output.pdf',
        deskew=True,
        languages=['eng'],
    )
    ocrmypdf.ocr(options)

{class}~ocrmypdf.OcrOptions is a Pydantic model that provides:

Full type hints and IDE autocompletion
Validation of option values at construction time
Clear documentation of all available options

{versionadded}

The {class}`~ocrmypdf.OcrOptions` class is now exported from the top-level
`ocrmypdf` module.

Legacy API

For compatibility with OCRmyPDF < v17, the traditional calling style with positional arguments is still fully supported:

python

import ocrmypdf

if __name__ == '__main__':  # To ensure correct behavior on Windows and macOS
    ocrmypdf.ocr('input.pdf', 'output.pdf', deskew=True)

With this style, all of the command line arguments are available and may be passed as equivalent keywords.

A few differences are that verbose and quiet are not available. Instead, output should be managed by configuring logging.

Parent process requirements

The {func}ocrmypdf.ocr function runs OCRmyPDF similar to command line execution. To do this, it will:

create worker processes or threads
manage the signal flags of its worker processes
execute other subprocesses (forking and executing other programs)

The Python process that calls {func}ocrmypdf.ocr() must be sufficiently privileged to perform these actions.

There currently is no option to manage how jobs are scheduled other than the argument jobs= which will limit the number of worker processes.

Creating a child process to call {func}ocrmypdf.ocr() is suggested. That way your application will survive and remain interactive even if OCRmyPDF fails for any reason. For example:

python

from multiprocessing import Process
import ocrmypdf
from ocrmypdf import OcrOptions


def ocrmypdf_process():
    options = OcrOptions(input_file='input.pdf', output_file='output.pdf')
    ocrmypdf.ocr(options)


def call_ocrmypdf_from_my_app():
    p = Process(target=ocrmypdf_process)
    p.start()
    p.join()

Programs that call {func}ocrmypdf.ocr() should also install a SIGBUS signal handler (except on Windows), to raise an exception if access to a memory mapped file fails. OCRmyPDF may use memory mapping.

Several OCR jobs may run concurrently in the same Python interpreter process.

Installing a plugin set mutates interpreter-global state, because of how OCRmyPDF's plugins and Python's library import system work, and a running job cannot survive that state being replaced underneath it. {func}ocrmypdf.ocr() therefore takes a readers-writer lock: installing a plugin set takes it exclusively, so it waits for every job already running, while a job holds it shared for its own run. A plugin set is installed once per interpreter and reused thereafter, so a job that asks for a plugin set some earlier job already installed never blocks.

The practical consequence is that concurrent jobs should all use the same plugin set. Jobs requesting different plugin sets serialize against each other. They must also use the same max_image_mpixels, since Pillow's decompression-bomb limit is interpreter-global and the last job to set it wins. If you need to run jobs with differing configurations, use separate processes.

Because a plugin set is installed only once, a plugin module given as a file path is executed once per interpreter rather than once per call. Plugins must not rely on being re-executed for each job, and must not store per-job state on the plugin manager, which concurrent jobs may share.

Note that N concurrent jobs each configured with jobs=M may spawn up to N*M workers, so size jobs accordingly.

:::{warning} On Windows and macOS, the script that calls {func}ocrmypdf.ocr() must be protected by an "ifmain" guard (if __name__ == '__main__'). If you do not take at least one of these steps, process semantics will prevent OCRmyPDF from working correctly. :::

Logging

OCRmyPDF will log under loggers named ocrmypdf. In addition, it imports pdfminer and PIL, both of which post log messages under those logging namespaces.

You can configure the logging as desired for your application or call {func}ocrmypdf.configure_logging to configure logging the same way OCRmyPDF itself does. The command line parameters such as --quiet and --verbose have no equivalents in the API; you must use the provided configuration function or do configuration in a way that suits your use case.

Progress monitoring

OCRmyPDF uses the rich package to implement its progress bars. {func}ocrmypdf.configure_logging will set up logging output to sys.stderr in a way that is compatible with the display of the progress bar. Use ocrmypdf.ocr(...progress_bar=False) to disable the progress bar.

Standard output

OCRmyPDF is strict about not writing to standard output so that users can safely use it in a pipeline and produce a valid output file. A caller application will have to ensure it does not write to standard output either, if it wants to be compatible with this behavior and support piping to a file. Another benefit of running OCRmyPDF in a child process, as recommended above, is that it will not interfere with the parent process's standard output.

Exceptions

OCRmyPDF may throw standard Python exceptions, ocrmypdf.exceptions.* exceptions, some exceptions related to multiprocessing, and {exc}KeyboardInterrupt. The parent process should provide an exception handler. OCRmyPDF will clean up its temporary files and worker processes automatically when an exception occurs.

When OCRmyPDF succeeds conditionally, it returns an integer exit code.

Plugin Development Changes

{versionchanged}

Plugin hooks now receive {class}`~ocrmypdf.OcrOptions` objects instead of
`argparse.Namespace`.

{class}~ocrmypdf.OcrOptions provides the same attribute access as Namespace (duck-typing compatible)
Plugin developers should update type hints: from ocrmypdf import OcrOptions
Built-in plugins no longer modify options in-place for better immutability

Most existing plugins will continue working without modification due to the duck-typing compatibility between {class}~ocrmypdf.OcrOptions and Namespace.