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, XFERINFOFUNCTION, 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. 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 documented below.

Error Reporting¶

PycURL callbacks are invoked as follows:

Python application -> perform() -> libcurl (C code) -> Python callback

Because callbacks are invoked by libcurl, they should not raise exceptions on failure but instead return appropriate values indicating failure. The documentation for individual callbacks below specifies expected success and failure return values.

Unhandled exceptions propagated out of Python callbacks will be intercepted by PycURL or the Python runtime. This will fail the callback with a generic failure status, in turn failing the perform() operation. A failing perform() will raise pycurl.error, but the error code used depends on the specific callback.

Rich context information like exception objects can be stored in various ways, for example the following example stores OPENSOCKET callback exception on the Curl object:

import pycurl, random, socket

class ConnectionRejected(Exception):
    pass

def opensocket(curl, purpose, curl_address):
    # always fail
    curl.exception = ConnectionRejected('Rejecting connection attempt in opensocket callback')
    return pycurl.SOCKET_BAD

    # the callback must create a socket if it does not fail,
    # see examples/opensocketexception.py

c = pycurl.Curl()
c.setopt(c.URL, 'http://pycurl.io')
c.exception = None
c.setopt(c.OPENSOCKETFUNCTION,
    lambda purpose, address: opensocket(c, purpose, address))

try:
    c.perform()
except pycurl.error as e:
    if e.args[0] == pycurl.E_COULDNT_CONNECT and c.exception:
        print(c.exception)
    else:
        print(e)

WRITEFUNCTION¶

WRITEFUNCTION(byte string) → number of characters written¶

Callback for writing data. Corresponds to CURLOPT_WRITEFUNCTION in libcurl.

On Python 3, the argument is of type bytes.

The WRITEFUNCTION callback may return the number of bytes written. If this number is not equal to the size of the byte string, this signifies an error and libcurl will abort the request. Returning None is an alternate way of indicating that the callback has consumed all of the string passed to it and, hence, succeeded.

write_test.py test shows how to use WRITEFUNCTION.

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()

HEADERFUNCTION¶

HEADERFUNCTION(byte string) → number of characters written¶

Callback for writing received headers. Corresponds to CURLOPT_HEADERFUNCTION in libcurl.

On Python 3, the argument is of type bytes.

The HEADERFUNCTION callback may return the number of bytes written. If this number is not equal to the size of the byte string, this signifies an error and libcurl will abort the request. Returning None is an alternate way of indicating that the callback has consumed all of the string passed to it and, hence, succeeded.

header_test.py test shows how to use WRITEFUNCTION.

READFUNCTION¶

READFUNCTION(number of characters to read) → byte string¶

Callback for reading data. Corresponds to CURLOPT_READFUNCTION in libcurl.

On Python 3, the callback must return either a byte string or a Unicode string consisting of ASCII code points only.

In addition, READFUNCTION may return READFUNC_ABORT or READFUNC_PAUSE. See the libcurl documentation for an explanation of these values.

The file_upload.py example in the distribution contains example code for using READFUNCTION.

SEEKFUNCTION¶

SEEKFUNCTION(offset, origin) → status¶: Callback for seek operations. Corresponds to CURLOPT_SEEKFUNCTION in libcurl.

IOCTLFUNCTION¶

IOCTLFUNCTION(ioctl cmd) → status¶

Callback for I/O operations. Corresponds to CURLOPT_IOCTLFUNCTION in libcurl.

Note: this callback is deprecated. Use SEEKFUNCTION instead.

DEBUGFUNCTION¶

DEBUGFUNCTION(debug message type, debug message byte string) → None¶

Callback for debug information. Corresponds to CURLOPT_DEBUGFUNCTION in libcurl.

Changed in version 7.19.5.2: The second argument to a DEBUGFUNCTION callback is now of type bytes on Python 3. Previously the argument was of type str.

debug_test.py test shows how to use DEBUGFUNCTION.

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, "https://curl.haxx.se/")
c.setopt(pycurl.VERBOSE, 1)
c.setopt(pycurl.DEBUGFUNCTION, test)
c.perform()

PROGRESSFUNCTION¶

PROGRESSFUNCTION(download total, downloaded, upload total, uploaded) → status¶

Callback for progress meter. Corresponds to CURLOPT_PROGRESSFUNCTION in libcurl.

PROGRESSFUNCTION receives amounts as floating point arguments to the callback. Since libcurl 7.32.0 PROGRESSFUNCTION is deprecated; XFERINFOFUNCTION should be used instead which receives amounts as long integers.

NOPROGRESS option must be set for False libcurl to invoke a progress callback, as PycURL by default sets NOPROGRESS to True.

XFERINFOFUNCTION¶

XFERINFOFUNCTION(download total, downloaded, upload total, uploaded) → status¶

Callback for progress meter. Corresponds to CURLOPT_XFERINFOFUNCTION in libcurl.

XFERINFOFUNCTION receives amounts as long integers.

NOPROGRESS option must be set for False libcurl to invoke a progress callback, as PycURL by default sets NOPROGRESS to True.

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 = pycurl.Curl()
c.setopt(c.URL, "http://slashdot.org/")
c.setopt(c.NOPROGRESS, False)
c.setopt(c.XFERINFOFUNCTION, progress)
c.perform()

OPENSOCKETFUNCTION¶

OPENSOCKETFUNCTION(purpose, address) → int¶

Callback for opening sockets. Corresponds to CURLOPT_OPENSOCKETFUNCTION in libcurl.

purpose is a SOCKTYPE_* value.

address is a namedtuple with family, socktype, protocol and addr fields, per CURLOPT_OPENSOCKETFUNCTION documentation.

addr is an object representing the address. Currently the following address families are supported:

AF_INET: addr is a 2-tuple of (host, port).
AF_INET6: addr is a 4-tuple of (host, port, flow info, scope id).
AF_UNIX: addr is a byte string containing path to the Unix socket.

Availability: Unix.

This behavior matches that of Python’s socket module.

The callback should return a socket object, a socket file descriptor or a Python object with a fileno property containing the socket file descriptor.

The callback may be unset by calling setopt with None as the value or by calling unsetopt.

open_socket_cb_test.py test shows how to use OPENSOCKETFUNCTION.

Changed in version 7.21.5: Previously, the callback received family, socktype, protocol and addr parameters (purpose was not passed and address was flattened). Also, AF_INET6 addresses were exposed as 2-tuples of (host, port) rather than 4-tuples.

Changed in version 7.19.3: addr parameter added to the callback.

CLOSESOCKETFUNCTION¶

CLOSESOCKETFUNCTION(curlfd) → int¶

Callback for setting socket options. Corresponds to CURLOPT_CLOSESOCKETFUNCTION in libcurl.

curlfd is the file descriptor to be closed.

The callback should return an int.

The callback may be unset by calling setopt with None as the value or by calling unsetopt.

close_socket_cb_test.py test shows how to use CLOSESOCKETFUNCTION.

SOCKOPTFUNCTION¶

SOCKOPTFUNCTION(curlfd, purpose) → int¶

Callback for setting socket options. Corresponds to CURLOPT_SOCKOPTFUNCTION in libcurl.

curlfd is the file descriptor of the newly created socket.

purpose is a SOCKTYPE_* value.

The callback should return an int.

The callback may be unset by calling setopt with None as the value or by calling unsetopt.

sockopt_cb_test.py test shows how to use SOCKOPTFUNCTION.

SSH_KEYFUNCTION¶

SSH_KEYFUNCTION(known_key, found_key, match) → int¶

Callback for known host matching logic. Corresponds to CURLOPT_SSH_KEYFUNCTION in libcurl.

known_key and found_key are instances of KhKey class which is a namedtuple with key and keytype fields, corresponding to libcurl’s struct curl_khkey:

KhKey = namedtuple('KhKey', ('key', 'keytype'))

On Python 2, the key field of KhKey is a str. On Python 3, the key field is bytes. keytype is an int.

known_key may be None when there is no known matching host key.

SSH_KEYFUNCTION callback should return a KHSTAT_* value.

The callback may be unset by calling setopt with None as the value or by calling unsetopt.

ssh_key_cb_test.py test shows how to use SSH_KEYFUNCTION.

TIMERFUNCTION¶

TIMERFUNCTION(timeout_ms) → None¶

Callback for installing a timer requested by libcurl. Corresponds to CURLMOPT_TIMERFUNCTION.

The application should arrange for a non-repeating timer to fire in timeout_ms milliseconds, at which point the application should call either socket_action or perform.

See examples/multi-socket_action-select.py for an example program that uses the timer function and the socket function.

SOCKETFUNCTION¶

SOCKETFUNCTION(what, sock_fd, multi, socketp) → None¶

Callback notifying the application about activity on libcurl sockets. Corresponds to CURLMOPT_SOCKETFUNCTION.

Note that the PycURL callback takes what as the first argument and sock_fd as the second argument, whereas the libcurl callback takes sock_fd as the first argument and what as the second argument.

The userp (“private callback pointer”) argument, as described in the CURLMOPT_SOCKETFUNCTION documentation, is set to the CurlMulti instance.

The socketp (“private socket pointer”) argument, as described in the CURLMOPT_SOCKETFUNCTION documentation, is set to the value provided to the assign method for the corresponding sock_fd, or None if no value was assigned.

See examples/multi-socket_action-select.py for an example program that uses the timer function and the socket function.

Callbacks¶

Error Reporting¶

WRITEFUNCTION¶

Example: Callbacks for document header and body¶

HEADERFUNCTION¶

READFUNCTION¶

SEEKFUNCTION¶

IOCTLFUNCTION¶

DEBUGFUNCTION¶

Example: Debug callbacks¶

PROGRESSFUNCTION¶

XFERINFOFUNCTION¶

Example: Download/upload progress callback¶

OPENSOCKETFUNCTION¶

CLOSESOCKETFUNCTION¶

SOCKOPTFUNCTION¶

SSH_KEYFUNCTION¶

TIMERFUNCTION¶

SOCKETFUNCTION¶

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