PySerial could not open port COM4: [Error 5] Access is denied - please help

Grant Edwards invalid at invalid.invalid
Thu Jun 28 01:24:57 CEST 2012

On 2012-06-27, Adam <adam at> wrote:
>> Actually, I believe someone in an earlier thread in the newsgroup or
>> elsewhere pointed out that serial ports automatically open under
>> Windows. I'd have to look it back up when I have the time, which I
>> don't have at the moment, unfortunately.

What they're referring to is that on startup, Windows used to open
serial ports and query them to see if there was a serial mouse
connected.  If it _thought_ it found a mouse, it would then hold the
port. I don't think that behavior has been enabled by default for a
long time.

If that were the case, then your terminal program wouldn't be able to
open the port either.

However, IIRC, some versions of windows do open and then close the
ports during the bus/device enumeration step of startup. However, they
don't keep the port open, so it doesn't affect the ability of user
applications to later open the port.

> Thanks, I think I read that as well but can't recall where.
> I am just running Python scripts (downloaded), which is not opening
> the serial port more than once (as Grant keeps assuming).

Well, I'm assuming your description of what you're doing is accurate.

If you're telling the truth, then the program is opening the port more
than once.

If the port wasn't already open, then calling ser.close() wouldn't do
_anything_.  Here's the close() implmentation from pyserial:

    def close(self):
        """Close port"""
        if self._isOpen:
            if self.hComPort:
                # Restore original timeout values:
                win32.SetCommTimeouts(self.hComPort, self._orgTimeouts)
                # Close COM-Port:
                self.hComPort = None
            self._isOpen = False
There's only _one_ place where self._isOpen is set to True, and that's
at the end of the open() call:

    def open(self):
        """Open port with current settings. This may throw a SerialException
           if the port cannot be opened."""
        self._overlappedWrite.hEvent = win32.CreateEvent(None, 0, 0, None)
        self._isOpen = True
If you have to add the call "ser.close()" before you can open the port
with "", then that means that the port _was_already_open_.

Grant Edwards               grant.b.edwards        Yow! World War III?
                                  at               No thanks!

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