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Pdb 'run/runeval' commands fails to check the type of
given argument.When argument to 'run/runeval' is
non-string the functions crashes with further
impilications on (correctly)invoking this functions: 
Python 2.5a1 (trunk:45527, Apr 18 2006, 11:12:31)
>>> def x(): pass
>>> import pdb
>>> pdb.run(x())
Traceback (most recent call last):
 File "<stdin>", line 1, in <module>
 File "/home/jkk/python/python-svn/Lib/pdb.py", line
1113, in run
 Pdb().run(statement, globals, locals)
 File "/home/jkk/python/python-svn/Lib/bdb.py", line
363, in run
 cmd = cmd+'\n'
TypeError: unsupported operand type(s) for +:
'NoneType' and 'str'
>>> pdb.run('x()')
> /home/jkk/python/python-svn/Lib/pdb.py(1113)run()
-> Pdb().run(statement, globals, locals)
(Pdb)
# CTRL-D pressed
Traceback (most recent call last):
 File "<stdin>", line 1, in <module>
 File "/home/jkk/python/python-svn/Lib/pdb.py", line
1113, in run
 Pdb().run(statement, globals, locals)
 File "/home/jkk/python/python-svn/Lib/pdb.py", line
1113, in run
 Pdb().run(statement, globals, locals)
 File "/home/jkk/python/python-svn/Lib/bdb.py", line
48, in trace_dispatch
 return self.dispatch_line(frame)
 File "/home/jkk/python/python-svn/Lib/bdb.py", line
67, in dispatch_line
 if self.quitting: raise BdbQuit
bdb.BdbQuit
The solution is to simply ensure that the first
argument passed to the 'run/runeval' functions is string.

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Patch is in #1472257 

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I would not classify your example as a crash. You passed a
wrong value (None in your case) into pdb.run() and got back
a TypeError...Isn't it an expected response?
E.g if you do: >>>max(13)
You'll also get: "TypeError"
Am I missing something? Could you clarify?

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The point is that when you want to invoke pdb.run correctly
(with string argument) after getting TypeError as in above
example,
your namespace is probably overwritten or deleted so that
you don't have access to previously defined symbols.
>>> def x():pass
>>> import pdb
>>> pdb.run(x())
TypeError
>>> pdb.run('x()')
> /home/jkk/python-svn/Lib/pdb.py(1122)run()
-> Pdb().run(statement, globals, locals)
(Pdb) pdb
*** NameError: name 'pdb' is not defined
(Pdb) x
*** NameError: name 'x' is not defined

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Well, I don't see anything in bdb's run which could
overwrite your namespace..
What seems to be happenning in your example is that bdb's
first line event happens while bdb is still in run(), so you
when you see pdb's prompt, are in bdb's namespace not in
yours.. If you do "next" you will get where you should be..
 bagira:~> python2.4
 Python 2.4.1 (#2, May 5 2005, 11:32:06) 
 >>> def x(): print "in x"
 >>> import pdb
 >>> pdb.run( x())
 in x
 TypeError: unsupported operand type(s) for +: 'NoneType'
and 'str'
 >>> pdb.run( 'x()' )
 > /usr/lib/python2.4/pdb.py(987)run()
 -> Pdb().run(statement, globals, locals)
 (Pdb) n #now you are back in your namespace
 > <string>(1)?()
 (Pdb) p x #and you get your symbols back
 <function x at 0x4021be2c>
 (Pdb) p pdb
 <module 'pdb' from '/usr/lib/python2.4/pdb.pyc'>
 (Pdb) n
 in x
 --Return--
 > <string>(1)?()->None
What do you think?

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You're right,but don't you think that this kind of switching
between namespaces could confuse users?

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I have looked at what's happening in a bit more detail and
now I agree that the current behaviour seems wrong.
Apparently, when one passes a nonstring to bdb's run(), bdb
sets tracing first and only then throws an exception...
Leaving tracing on seems like a bad thing to do 
Would calling settrace() a couple lines later be the
simplest solution then?
so relevant fragment of bdb.py's run() becomes:
 if not isinstance(cmd, types.CodeType):
 cmd = cmd+'\n'
 sys.settrace(self.trace_dispatch)
This should prevent bdb from turning tracing too early.
What do you think?

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Is your comment directly related to this bug?
Maybe you should create separete bug tracker item to discuss
problem presented in your comment?

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Simple solution is to catch TypeError in bdb's run function:
 try:
 if not isinstance(cmd, types.CodeType):
 cmd = cmd+'\n'
 except TypeError:
 pass
Now it seems that handling nonstring arguments is better:
>>> import pdb
>>> def x():pass
...
>>> pdb.run(x())
Traceback (most recent call last):
 File "<stdin>", line 1, in <module>
 File "/home/jkk/python-svn/Lib/pdb.py", line 1122, in run
 Pdb().run(statement, globals, locals)
 File "/home/jkk/python-svn/Lib/bdb.py", line 369, in run
 exec cmd in globals, locals
TypeError: exec: arg 1 must be a string, file, or code object
>>> pdb.run('x()')
> <string>(1)<module>()
(Pdb) x
<function x at 0x4024a9cc>
(Pdb) pdb.run('x()')
(Pdb)
>>>

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>> Is your comment directly related to this bug?
Yes, my suggestion was to call settrace() after 
 cmd = cmd+'\n'
which should solve the original (bdb leaving tracing on)
problem.. Your latest suggestion would do the same thing by
different means..
However, I think there an even better way: can we simply remove
- if not isinstance(cmd, types.CodeType):
- cmd = cmd+'\n'
from bdb.py???
This would both solve the original problem and make error
message more meaningful IMO..
 > exec: arg 1 must be a string, file, or code object
Rather than:
 >Unsupported operand type(s) for +: 'NoneType' and 'str'
This seems to work for me, would you be interested in
testing and submitting a patch for this?
A bit more information: it appears that older versions of
python in some cases required string passed to exec to end
with '\n', this was even mentioned in python FAQ.. E.g if I
feed 
 
 exec 'for i in range(4):\nx=i'
into python2.1, I get a Syntax error..
However, this does work in python2.3.3, 2.4,2.5...
So was this + '\n' indeed a workaround for a deficiency in exec?
apparently older versions of python required the string
passed to exec to end with '\n'. It was even mentioned in
older FAQs... However, latest versions of python seem to
accept strings without new line... Eg. even

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I think your solution works for me too;)I attached patch
which modify run and runeval function according to your
suggestion.
I tested both run and runeval functions with string and code
object as 
an argument,and also run with file object.
It appears that patch works fine.

Issue 1472257 was rejected.

Updated patch with tests and documentation.It fixes the issue by leaving
argument checking to exec and eval functions.The result is a more
readable error messages when invoking run/runeval with incorrect
arguments (see discussion above) and also ability to run pdb on an open
file objects.

BTW, patch is based on trunk.

Committed in trunk r83266; file objects are no longer allowed for exec(), therefore I dropped mentions of that.