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Python interface to libgphoto2

Project description

python-gphoto2 is a comprehensive Python interface (or binding) to libgphoto2. It is built using SWIG to automatically generate the interface code. This gives direct access to nearly all the libgphoto2 functions, but sometimes in a rather un-Pythonic manner.

Other Python bindings to libgphoto2 are available: piggyphoto uses ctypes (included in standard Python installations) to interface to the library. The gphoto2 source tree includes some Python bindings which also use ctypes. gphoto2-cffi uses cffi.

Dependencies

Note that you need the “development headers” versions of libgphoto2 and Python. Most Linux distributions’ package managers have these, but the names vary. Look for libgphoto2-2-dev or libgphoto2-devel or something similar.

Installation and testing

There are several ways to install python-gphoto2, with varying levels of control over the installation process. You can install it with pip, or by downloading an archive, or by getting the source from GitHub.

The commands below will install python-gphoto2 for your default Python version. To install for both Python 2 and Python 3, run the installation process twice with specific commands, i.e. pip2 and pip3 or python2 and python3.

Install with pip

The easiest installation method is to use the pip command:

sudo pip install -v gphoto2

Note that this may take longer than you expect as the package’s modules are compiled during installation. The -v option increases pip’s verbosity so you can see that it’s doing something.

Install a downloaded archive

Visit PyPI and download one of the zip or tar.gz files, then extract it and change to the new directory. For example:

tar xzf gphoto2-1.3.4.tar.gz
cd gphoto2-1.3.4

Python’s distutils are used to build and install python-gphoto2:

python setup.py build
sudo python setup.py install

Install from GitHub (SWIG required)

SWIG (http://swig.org/) should be installable via your operating system’s package manager. Note that this may be an older version of SWIG than the one used to generate the files on PyPI.

To install the current development version, use git to “clone” the GitHub repository, then change to the new directory:

git clone https://github.com/jim-easterbrook/python-gphoto2.git
cd python-gphoto2

As before, Python’s distutils are used to build and install python-gphoto2, but now you have to run SWIG first to generate the files to be compiled:

python setup.py build_swig
python setup.py build
sudo python setup.py install

See “running SWIG” below for more detail.

Testing

Connect a digital camera to your computer, switch it on, and try one of the example programs:

python examples/camera-summary.py

If this works then you’re ready to start using python-gphoto2.

Reinstalling

If you update or move your installation of libgphoto2 the Python gphoto2 package may fail to import one of the libgphoto2 shared object files. If this happens you need to rebuild and reinstall the Python gphoto2 package:

sudo pip install -v -U --force-reinstall gphoto2

if you installed with pip, or

rm -rf build
python setup.py build
sudo python setup.py install

if you installed from source.

Using python-gphoto2

The Python interface to libgphoto2 should allow you to do anything you could do in a C program. However, there are still bits missing and functions that cannot be called from Python. Let me know if you run into any problems.

The following paragraphs show how the Python interfaces differ from C. See the example programs for typical usage of the Python gphoto2 API.

“C” interface

Using SWIG to generate the Python interfaces automatically means that every function in libgphoto2 should be available to Python. The pydoc command can be used to show basic information about a function:

jim@firefly ~/python-gphoto2 $ pydoc gphoto2.gp_camera_folder_list_files
Help on built-in function gp_camera_folder_list_files in gphoto2:

gphoto2.gp_camera_folder_list_files = gp_camera_folder_list_files(...)
    gp_camera_folder_list_files(camera, folder, context) -> int

    Parameters:
        camera: Camera *
        folder: char const *
        context: Context *


    See also: gphoto2.Camera.folder_list_files

jim@firefly ~/python-gphoto2 $

If you compare this to the C API documentation of gp_camera_folder_list_files you will see that the C function signature includes an additional parameter list of type CameraList *. This is an “output” parameter, a concept that doesn’t really exist in Python. The Python version of gp_camera_folder_list_files returns a sequence containing the integer error code and the list value.

Most of the libgphoto2 functions that use pointer parameters to return values in the C API have been adapted like this in the Python API. (Unfortunately I’ve not found a way to persuade SWIG to include this extra return value in the documentation. You should use pydoc to check the parameters expected by the Python function.)

For example, the C code:

#include "gphoto2.h"
int error;
Camera *camera;
error = gp_camera_new(&camera);
...
error = gp_camera_unref(camera);

has this Python equivalent:

import gphoto2 as gp
error, camera = gp.gp_camera_new()
...

Note that the gp_camera_unref() call is not needed. It is called automatically when the Python camera object is deleted.

Here is a complete example program (without any error checking):

import gphoto2 as gp
context = gp.gp_context_new()
error, camera = gp.gp_camera_new()
error = gp.gp_camera_init(camera, context)
error, text = gp.gp_camera_get_summary(camera, context)
print('Summary')
print('=======')
print(text.text)
error = gp.gp_camera_exit(camera, context)

“Object oriented” interface

Many of the libgphoto2 functions have been added as methods of the appropriate GPhoto2 object. This allows GPhoto2 to be used in a more “Pythonic” style. These methods also include error checking. If an error occurs they raise a Python GPhoto2Error exception.

The example program can be re-written as follows:

import gphoto2 as gp
context = gp.Context()
camera = gp.Camera()
camera.init(context)
text = camera.get_summary(context)
print('Summary')
print('=======')
print(str(text))
camera.exit(context)

The object methods are more “hand crafted” than the rest of the Python bindings, which are automatically generated from the library header files. This means that there are some functions in the “C” interface that do not have corresponding methods. Those that do include a “see also” reference in their docstring, as shown in the pydoc example above.

Error checking

Most of the libgphoto2 functions return an integer to indicate success or failure. The Python interface includes a check_result() function to check these values and raise a GPhoto2Error exception if an error occurs.

This function also removes the error code from lists such as that returned by gp_camera_new() in the example. Using this function the earlier example becomes:

import gphoto2 as gp
context = gp.gp_context_new()
camera = gp.check_result(gp.gp_camera_new())
gp.check_result(gp.gp_camera_init(camera, context))
text = gp.check_result(gp.gp_camera_get_summary(camera, context))
print('Summary')
print('=======')
print(text.text)
gp.check_result(gp.gp_camera_exit(camera, context))

There may be some circumstances where you don’t want an exception to be raised when some errors occur. You can “fine tune” the behaviour of the check_result() function by adjusting the error_severity variable:

import gphoto2 as gp
gp.error_severity[gp.GP_ERROR] = logging.WARNING
...

In this case a warning message will be logged (using Python’s standard logging module) but no exception will be raised when a GP_ERROR error occurs. However, this is a “blanket” approach that treats all GP_ERROR errors the same. It is better to test for particular error conditions after particular operations, as described below.

The GPhoto2Error exception object has two attributes that may be useful in an exception handler. GPhoto2Error.code stores the integer error generated by the library function and GPhoto2Error.string stores the corresponding error message.

For example, to wait for a user to connect a camera you could do something like this:

import gphoto2 as gp
...
print('Please connect and switch on your camera')
while True:
    try:
        camera.init(context)
    except gp.GPhoto2Error as ex:
        if ex.code == gp.GP_ERROR_MODEL_NOT_FOUND:
            # no camera, try again in 2 seconds
            time.sleep(2)
            continue
        # some other error we can't handle here
        raise
    # operation completed successfully so exit loop
    break
# continue with rest of program
...

When just calling a single function like this, it’s probably easier to test the error value directly instead of using Python exceptions:

import gphoto2 as gp
...
print('Please connect and switch on your camera')
while True:
    error = gp.gp_camera_init(camera, context)
    if error >= gp.GP_OK:
        # operation completed successfully so exit loop
        break
    if error != gp.GP_ERROR_MODEL_NOT_FOUND:
        # some other error we can't handle here
        raise gp.GPhoto2Error(error)
    # no camera, try again in 2 seconds
    time.sleep(2)
# continue with rest of program
...

Logging

The libgphoto2 library includes functions (such as gp_log()) to output messages from its various functions. These messages are mostly used for debugging purposes, and it can be helpful to see them when using libgphoto2 from Python. The Python interface includes a use_python_logging() function to connect libgphoto2 logging to the standard Python logging system. You should call use_python_logging() near the start of your program, as shown in the examples.

The libgphoto2 logging messages have four possible severity levels, each of which is mapped to a suitable Python logging severity. You can override this mapping by passing your own to use_python_logging():

import logging
import gphoto2 as gp
...
gp.use_python_logging(mapping={
    gp.GP_LOG_ERROR   : logging.INFO,
    gp.GP_LOG_VERBOSE : logging.DEBUG,
    gp.GP_LOG_DEBUG   : logging.DEBUG - 3,
    gp.GP_LOG_DATA    : logging.DEBUG - 6})
...

If you prefer to use your own logging system you can define a logging callback function in Python. The function must take 3 parameters: level, domain and string. Since python-gphoto2 version 1.3 the callback function is installed with gp_log_add_func (previously called gp_log_add_func_py):

import gphoto2 as gp
...
def callback(level, domain, string):
    print('Callback: level =', level, ', domain =', domain, ', string =', string)
...
callback_id = gp.check_result(gp.gp_log_add_func(gp.GP_LOG_VERBOSE, callback))
...

Since python-gphoto2 version 1.4 you can pass some user data to your callback function (e.g. to log which thread an error occurred in):

import gphoto2 as gp
...
def callback(level, domain, string, data=None):
    print('Callback: level =', level, ', domain =', domain, ', string =', string, 'data =', data)
...
callback_id1 = gp.check_result(gp.gp_log_add_func(gp.GP_LOG_VERBOSE, callback))
callback_id2 = gp.check_result(gp.gp_log_add_func(gp.GP_LOG_VERBOSE, callback, 123))
...

What to do if you have a problem

If you find a problem in the Python gphoto2 interface (e.g. a segfault, a missing function, or a function without a usable return value) then please report it on the GitHub “issues” page (https://github.com/jim-easterbrook/python-gphoto2/issues) or email jim@jim-easterbrook.me.uk.

If your problem is more general, e.g. difficulty with capturing multiple images, then try doing what you want to do with the gphoto2 command line program. If the problem persists then it might be worth asking on the gphoto-user mailing list. Another reader of the mailing list may have the same camera model and already know what to do.

Notes on some gphoto2 functions

gp_file_get_data_and_size / CameraFile.get_data_and_size

Since python-gphoto2 version 1.2.0 these functions return a FileData object that supports the buffer protocol. The data can be made accessible to Python (2.7 and 3.x) by using a memoryview object. This allows the data to be used without copying. See the copy-data.py example for typical usage.

In earlier versions of python-gphoto2 these functions returned a str (Python 2) or bytes (Python 3) object containing a copy of the data in the CameraFile object.

gp_camera_file_read / Camera.file_read / gp_file_slurp / CameraFile.slurp

Although the documentation says the buf parameter is of type char * you can pass any Python object that exposes a writeable buffer interface. This allows you to read a file directly into a Python object without additional copying. See the copy-chunks.py example which uses memoryview to expose a bytearray.

gp_camera_wait_for_event / Camera.wait_for_event

These functions return both the event type and the event data. The data you get depends on the type. GP_EVENT_FILE_ADDED and GP_EVENT_FOLDER_ADDED events return a CameraFilePath, others return None.

Running SWIG

SWIG is used to convert the .i interface definition files in src/gphoto2 to .py and .c files. These are then compiled to build the Python interface to libgphoto2. The files downloaded from PyPI include the SWIG generated files, but you may wish to regenerate them by running SWIG again (e.g. to test a new version of SWIG or of libgphoto2). You will also need to run SWIG if you have downloaded the python-gphoto2 sources from GitHub instead of using PyPI.

The file setup.py defines an extra command to run SWIG. It has no user options:

python setup.py build_swig

By default this builds the interface for the version of libgphoto2 installed on your computer. The interface files are created in directories with names like src/swig-bi-gp2.5-py3. This naming scheme allows for different versions of Python and libgphoto2, and use (or not) of the SWIG -builtin flag. The appropriate version is chosen when the interface is built.

To build interfaces for additional versions of libgphoto2 (e.g. v2.4 as well as v2.5) you need to put a copy of that version’s include (.h) files in a sub-directory of your working directory called include/gphoto2-2.x and then run setup.py build_swig again.

Licence

python-gphoto2 - Python interface to libgphoto2
Copyright (C) 2014-16 Jim Easterbrook jim@jim-easterbrook.me.uk

This program is free software: you can redistribute it and/or modify it under the terms of the GNU General Public License as published by the Free Software Foundation, either version 3 of the License, or (at your option) any later version.

This program is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License for more details.

You should have received a copy of the GNU General Public License along with this program. If not, see http://www.gnu.org/licenses/.

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