>>> "%.%s" % ()
'%s'
>>> "%(a).%(b)s" % dict(a=2)
'%(b)s'
>>> "%(a).%(b)s" % dict(a=2, b=3)
'%(b)s'
>>> "%(a).%(b)s" % dict()
Traceback (most recent call last):
  File "<pyshell#6>", line 1, in -toplevel-
    "%(a).%(b)s" % dict()
KeyError: 'a'

this is counter intuitive and cannot be deduced from the documentation.

It's straightforward if you consider the implementation of the
requirement that %% renders a single percent sign: the second % is
parsed just like any other formatting code (i, d, f, etc.) and the stuff
between the first % and the formatting code is treated completely uniformly.

The main problem with this is that the following code does not make any
sense:
"%(a)%" % dict(a=3)

It has no semantic meaning (take the dictionary paramater a, and do nothing
with it).
It must be a user bug (except in very wierd cases).

I agree that when I consider the implementaion, it makes sense, but as a
python user, this behavior is really non-intuitive.

2008/8/7 Guido van Rossum <report@bugs.python.org>

Guido van Rossum <guido@python.org> added the comment:

It's straightforward if you consider the implementation of the
requirement that %% renders a single percent sign: the second % is
parsed just like any other formatting code (i, d, f, etc.) and the stuff
between the first % and the formatting code is treated completely
uniformly.

----------
nosy: +gvanrossum
resolution: -> rejected
status: open -> closed

---

Python tracker <report@bugs.python.org>
<http://bugs.python.org/issue3516\>

---

I'd rather see it this way: It is a programming error if a format string
contains a reference to a nonexisting dictionary key, no matter what
formatting specifier is used. The implementation is quite consistent here.