Python socket connect() vs. connect_ex()

Question

I would like to take a look at those 2 functions in Python. I did some research and bumped into something like this:

socket.connect_ex(address)

Like connect(address), but return an error indicator instead of raising an exception for errors returned by the C-level connect() call (other problems, such as “host not found,” can still raise exceptions). The error indicator is 0 if the operation succeeded, otherwise the value of the errno variable. This is useful to support, for example, asynchronous connects.

I totally get this. I wrote a simple port scanner to test efficiency and got interesting results:

[*] start scanning on host 127.0.0.1 for ports range(0, 65535) with 
method(connect)
[*] method connect finished in 0:00:12.253352
[*] start scanning on host 127.0.0.1 for ports range(0, 65535) with 
method(connect_ex)
[*] method connect_ex finished in 0:00:06.839319

So, connect_ex seems to be twice as efficient as connect. If it comes to syntax the only difference is that with connect you surround it by try except and catch an error when it fails to connect and with connect_ex you check the result for being 0 or errno. That is all clear but my question is why connect_ex works faster than connect? What reason is down there causing such behaviour?

Answer 1

socket.connect(address) -> Connect to a remote socket at address. (The format of address depends on the address family — see above.)

If the connection is interrupted by a signal, the method waits until the connection completes, or raise a socket.timeout on timeout, if the signal handler doesn’t raise an exception and the socket is blocking or has a timeout. For non-blocking sockets, the method raises an InterruptedError exception if the connection is interrupted by a signal (or the exception raised by the signal handler).

Raises an auditing event socket.connect with arguments self, address.

Changed in version 3.5: The method now waits until the connection completes instead of raising an InterruptedError exception if the connection is interrupted by a signal, the signal handler doesn’t raise an exception and the socket is blocking or has a timeout (see the PEP 475 for the rationale).

socket.connect_ex(address) -> Like connect(address), but return an error indicator instead of raising an exception for errors returned by the C-level connect() call (other problems, such as “host not found,” can still raise exceptions). The error indicator is 0 if the operation succeeded, otherwise the value of the errno variable. This is useful to support, for example, asynchronous connects.

Raises an auditing event socket.connect with arguments self, address.

Ref: https://docs.python.org/3/library/socket.html