How can I find out the name of some variable?

edit

You should understand that an object may have different names:

sage: p = 3
sage: q = p
sage: id(p)
529391024
sage: id(q)
529391024

Here, p is the name of the same object than q (i mean, they both are the name of the same location in memory). Hence, your question is not well defined, since Sage (or Python) will not be able to make a difference between p and q. If you need such a feature, there may a better way to write your progam.

That said, you can try to look for your variable in the globals() dictionary:

sage: def my_name(p):
....:     for name, value in globals().items():
....:         if id(value) == id(p):
....:             return name

sage: my_name(p)
'p'
sage: my_name(q)
'p'

That said, this is quite a dirty method, and you should consider avoiding it.