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 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, WRITEFUNCTION cannot be used with WRITEDATA, READFUNCTION cannot be used with READDATA, HEADERFUNCTION cannot be used with WRITEHEADER, PROGRESSFUNCTION cannot be used with PROGRESSDATA, and DEBUGFUNCTION cannot be used with DEBUGDATA. 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 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

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()

$Id: callbacks.html,v 1.12 2004/03/16 10:13:33 kjetilja Exp $