sax.saxutils.escape -- always escapes <, >, &, even
if entities are provided.  

E.g. 
escape("<abcd>", {'a':'\a'}) would produce

"<\abcd%rt;" instead of "<\abcd>"

While it might be expected behaviour, the documentation
states 

"You can escape other strings of data by passing a
dictionary as the optional entities parameter. The keys
and values must all be strings; each key will be
replaced with its corresponding value. "

and it is not at all obvious that entities specify
_additional_ (to <, >, &) escape strings.

It is also impossible to do custom escapes of <,>,&
using saxutils.escape. 
(while custom escapes of < and > might be never
necessary for XML,  the escape function is general
enough to be of interest in other circumstances).

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The behaviour is indeed intentional; reclassifying this as a
patch.

I've included a unified diff that explicitly states the behavior of &,
<, and > for escape/unescape in the documentation. It's based on
msandler's patch.

Fixed in r60099.