PyCurl: Callbacks

# Callbacks

For more fine-grained control, libcurl allows a number of callbacks to be associated with each connection. In pycurl, callbacks are defined using the `setopt()` method for Curl objects with options WRITEFUNCTION, READFUNCTION, HEADERFUNCTION, PROGRESSFUNCTION, IOCTLFUNCTION, or DEBUGFUNCTION. These options correspond to the libcurl options with CURLOPT_* prefix removed. A callback in pycurl must be either a regular Python function, a class method or an extension type function.

There are some limitations to some of the options which can be used concurrently with the pycurl callbacks compared to the libcurl callbacks. This is to allow different callback functions to be associated with different Curl objects. More specifically, WRITEDATA cannot be used with WRITEFUNCTION, READDATA cannot be used with READFUNCTION, WRITEHEADER cannot be used with HEADERFUNCTION, PROGRESSDATA cannot be used with PROGRESSFUNCTION, IOCTLDATA cannot be used with IOCTLFUNCTION, and DEBUGDATA cannot be used with DEBUGFUNCTION. In practice, these limitations can be overcome by having a callback function be a class instance method and rather use the class instance attributes to store per object data such as files used in the callbacks.

The signature of each callback used in pycurl is as follows:
`WRITEFUNCTION(`string`)` -> number of characters written
`READFUNCTION(`number of characters to read`)`-> string
`HEADERFUNCTION(`string`)` -> number of characters written
`PROGRESSFUNCTION(`download total, downloaded, upload total, uploaded`)` -> status
`DEBUGFUNCTION(`debug message type, debug message string`)` -> None
`IOCTLFUNCTION(`ioctl cmd`)` -> status

In addition, `READFUNCTION` may return `READFUNC_ABORT` or `READFUNC_PAUSE`. See the libcurl documentation for an explanation of these values. The `WRITEFUNCTION` and `HEADERFUNCTION` callbacks may return `None`, which is an alternate way of indicating that the callback has consumed all of the string passed to it.

## Example: Callbacks for document header and body

This example prints the header data to stderr and the body data to stdout. Also note that neither callback returns the number of bytes written. For WRITEFUNCTION and HEADERFUNCTION callbacks, returning None implies that all bytes where written.

        ## Callback function invoked when body data is ready
        def body(buf):
            # Print body data to stdout
            import sys
            sys.stdout.write(buf)
            # Returning None implies that all bytes were written

        ## Callback function invoked when header data is ready
        def header(buf):
            # Print header data to stderr
            import sys
            sys.stderr.write(buf)
            # Returning None implies that all bytes were written

        c = pycurl.Curl()
        c.setopt(pycurl.URL, "http://www.python.org/")
        c.setopt(pycurl.WRITEFUNCTION, body)
        c.setopt(pycurl.HEADERFUNCTION, header)
        c.perform()

## Example: Download/upload progress callback

This example shows how to use the progress callback. When downloading a document, the arguments related to uploads are zero, and vice versa.

        ## Callback function invoked when download/upload has progress
        def progress(download_t, download_d, upload_t, upload_d):
            print "Total to download", download_t
            print "Total downloaded", download_d
            print "Total to upload", upload_t
            print "Total uploaded", upload_d

        c.setopt(c.URL, "http://slashdot.org/")
        c.setopt(c.NOPROGRESS, 0)
        c.setopt(c.PROGRESSFUNCTION, progress)
        c.perform()

## Example: Debug callbacks

This example shows how to use the debug callback. The debug message type is an integer indicating the type of debug message. The VERBOSE option must be enabled for this callback to be invoked.

        def test(debug_type, debug_msg):
            print "debug(%d): %s" % (debug_type, debug_msg)

        c = pycurl.Curl()
        c.setopt(pycurl.URL, "http://curl.haxx.se/")
        c.setopt(pycurl.VERBOSE, 1)
        c.setopt(pycurl.DEBUGFUNCTION, test)
        c.perform()

## Other examples

The pycurl distribution also contains a number of test scripts and examples which show how to use the various callbacks in libcurl. For instance, the file 'examples/file_upload.py' in the distribution contains example code for using READFUNCTION, 'tests/test_cb.py' shows WRITEFUNCTION and HEADERFUNCTION, 'tests/test_debug.py' shows DEBUGFUNCTION, and 'tests/test_getinfo.py' shows PROGRESSFUNCTION.