Disabling 'http_proxy' not working as expected (was: Transport error: Server refuses to fullfil the request)

[Ben Finney]

Ben Finney <bignose+hates-spam at benfinney.id.au> writes:

> In a later message in this thread, I demonstrated the same problem
> occuring with an empty 'http_proxy' veriable.

Except apparently I didn't.

Running

    $ http_proxy= bzr -Dhttp -Dtransport branch http://people.debian.org/~mvo/bzr/python-apt/python-apt--debian-sid/

the log shows that it's *still* using my proxy.

If instead I run

    $ unset http_proxy
    $ export http_proxy
    $ bzr -Dhttp -Dtransport branch http://people.debian.org/~mvo/bzr/python-apt/python-apt--debian-sid/

the log then shows the proxy is not used at all.

Why would the first form not have the effect of "don't use the proxy",
when the second form does? Every program I've ever needed to set or
disable an environment variable, setting it on the commandline as in
the first form works fine. Why is bzr behaving differently to this?