Changing behaviour of namespaces

[Mikael Olofsson]

Hi!

This is in Python 2.3.4 under WinXP.

I have a situation where I think changing the behaviour of a namespace
would be very nice. The goal is to be able to run a python file from
another in a separate namespace in such a way that a NameError is not
raised if a non-existing variable is used in the file. The file that I
want to run is taken out of context, and some needed variables may be
missing that are normally there when used in the intended context. OK!
When I run it out of context, I do not want the execution to stop
half-way through the file based on NameErrors, instead a predefined
dummy variable should be used. The question is: Can I achieve that?

Google didn't give me much, so I experimented a bit: First, I created a
dummy object, which is meant to be used when a variable is missing:

class dummyClass(obje ct):
pass
dummyObject = dummyClass()

The actual dummy object is supposed to be a bit more complicated than
that. The next step was to subclass dict:

class forgivingDict(d ict):
def __init__(self,* args,**kwargs):
dict.__init__(s elf,*args,**kwa rgs)
def __getitem__(sel f,key):
print ' Getting', key
try:
return dict.__getitem_ _(self,key)
except KeyError:
return dummyObject
def __setitem__(sel f,key,value):
print ' Setting', key, 'to', value
dict.__setitem_ _(self,key,valu e)
def __contains__(se lf,item):
return True
def __repr__(self):
return 'forgivingDict( %s)' % (dict.__repr__( self),)

Then I tried to execute a file using a forgivingDict() as namespace:

ns = forgivingDict()
execfile(filePa th, ns)

A file containing the following passes perfectly OK:

a = 0
b = a

But a file containing the following produces a NameError:

b = a

The error traceback for the interested:

Traceback (most recent call last):
File "//charm/mikael/My
Documents/Programmering/Python/Tester/voidTest.py", line 130, in ?
execfile('test2 .py', ns)
File "test2.py", line 1, in ?
b = a
NameError: name 'a' is not defined

Now, that is exactly what is to expect if ns is a dict(), but I was
hoping that b would have ended up as dummyObject when ns is a
forgivingDict() . One thing: Nothing is printed out when execfile-ing the
files, which means that the methods forgivingDict._ _getitem__ and
forgivingDict._ _setitem__ are never used. My guess was that the
execution done by execfile uses dict.__getitem_ _(ns, key) and
dict.__setitem_ _(ns,key,value) instead of ns[key] and ns[key]=value when
getting and setting variables. That is probably intentional from the
developers and done that way for a good reason.

Alternative test: I've tried to replace __builtin__.dic t with my
forgivingDict, slightly modified to make sure that the inheritance still
works, which didn't change anything. So it seams that the execution done
by execfile does *not* use dict.__getitem_ _(ns, key) and
dict.__setitem_ _(ns,key,value) after all, but something else. Probably
the original dict is around somewhere else, and execfile uses the
corresponding methods of that.

My approach seems flawed. Any ideas? Can I somehow execute a complete
file even if there are non-existent variables used somewhere? A
try-except around the execfile statement is not an alternative, since
that stops the execution where the error occurs.

Should I perhaps read up on the compiler module? I mean: Using the
compiler module to analyze the content of the file, and based on that
updating my namespace before executing the file.

/MiO

[Peter Otten]

Re: Changing behaviour of namespaces

Mikael Olofsson wrote:

This is in Python 2.3.4 under WinXP.

To feed an arbitrary mapping object to execfile() you need to upgrade to
Python 2.4.

http://docs.python.org/dev/lib/built-in-funcs.html#l2h-26

Peter

[Mikael Olofsson]

Re: Changing behaviour of namespaces - solved, I think

Peter Otten wrote:

To feed an arbitrary mapping object to execfile() you need to upgrade to
Python 2.4.

Thanks! As clear as possible. I will do that.

FYI: I think I managed to achieve what I want in Py2.3 using the
compiler module:

def getNamesFromAst Node(node,varSe t):
if node.__class__ in (compiler.ast.G lobal,compiler. ast.Import):
varSet.union_up date(node.names )
if node.__class__ in (compiler.ast.N ame,):
varSet.add(node .name)
for subNode in node.getChildNo des():
getNamesFromAst Node(subNode,va rSet)

# Get all variable names that are referenced in the file:
varSet = sets.Set()
mainNode = compiler.parseF ile(filePath)
getNamesFromAst Node(mainNode,v arSet)

# Define a namespace and update it:
ns = {}
for varName in varSet:
ns[varName] = dummyObject

# Execute the file without NameErrors:
execfile(filePa th,ns)

Batteries included!

/MiO

[Peter Otten]

Re: Changing behaviour of namespaces - solved, I think

Mikael Olofsson wrote:

Peter Otten wrote:

>To feed an arbitrary mapping object to execfile() you need to upgrade to
>Python 2.4.

>
Thanks! As clear as possible. I will do that.
>
FYI: I think I managed to achieve what I want in Py2.3 using the
compiler module:
>
def getNamesFromAst Node(node,varSe t):
if node.__class__ in (compiler.ast.G lobal,compiler. ast.Import):
varSet.union_up date(node.names )
if node.__class__ in (compiler.ast.N ame,):
varSet.add(node .name)
for subNode in node.getChildNo des():
getNamesFromAst Node(subNode,va rSet)
>
# Get all variable names that are referenced in the file:
varSet = sets.Set()
mainNode = compiler.parseF ile(filePath)
getNamesFromAst Node(mainNode,v arSet)
>
# Define a namespace and update it:
ns = {}
for varName in varSet:
ns[varName] = dummyObject
>
# Execute the file without NameErrors:
execfile(filePa th,ns)
>
Batteries included!

Clearly more elegant than:

>>print open(filename). read()

b = a

>>dummy = 42
>>names = []
>>while 1:

.... ns = dict((n, dummy) for n in names)
.... try:
.... execfile(filena me, ns)
.... except NameError, e:
.... names.append(e[0][6:-16])
.... else:
.... break
....

>>names

['a']

:-)

Peter

[Mikael Olofsson]

Re: Changing behaviour of namespaces - solved, I think

Peter Otten wrote:

Clearly more elegant than:
>
>

>>>print open(filename). read()
>>>>

b = a
>

>>>dummy = 42
>>>names = []
>>>while 1:
>>>>

... ns = dict((n, dummy) for n in names)
... try:
... execfile(filena me, ns)
... except NameError, e:
... names.append(e[0][6:-16])
... else:
... break
...
>

>>>names
>>>>

['a']
>

That sure is nicer. My solution looks like shooting mosquitoes with an
elephant rifle i comparison. Thanks.

/MiO