known the attribute of a class how to get the class in python

You can't, because - just like in the real world - there can be multiple people with the name 'bob'.

This assumes you have some way of knowing who all of your people are. Let's say you have a list of people, and you want to find 'bob'.

a = person('anne', 'female')
b = person('bob', 'male')
c = person('cathy', 'female')

people = [a, b, c]

Now you can check each person to see if their name is 'bob'.

maybe_bob = [pers for pers in people if pers.name == 'bob']

If 'bob' exists, we can access his object by calling the first item in the list.

try:
    bob = maybe_bob[0]
except IndexError:
    print "No one named 'bob'."

Of course, like wim said, this will break down if there are multiple people named 'bob' in your list.

Since bob is unique you can get the instance with ast and pull the gender from that:

import inspect
import importlib
import ast   

class FindAttr(ast.NodeVisitor):
    def __init__(self, name):
        self.name = name
        self.b = None

    def visit_Assign(self, node):
        for t in node.targets:
            if isinstance(t, ast.Attribute) and hasattr(node.value, "s"):
                if node.value.s == self.name:
                    self.b = (getattr(mod,t.value.id))
                    return


mod = "test"
mod = importlib.import_module(mod)
p = ast.parse(inspect.getsource(mod))

f = FindAttr("bob")
f.visit(p)

using your example in test.py:

In [4]: mod = "test"

In [5]: mod = importlib.import_module(mod)

In [6]: p = ast.parse(inspect.getsource(mod))

In [7]: f = FindAttr("bob")

In [8]: f.visit(p)


In [9]: print(f.b)
<test.person instance at 0x7f2a78e623f8>
In [10]: print(f.b.gender)
male

In [11]: print(f.b.name)
bob
In [12]: print(f.b.__class__)
test.person

having a dict for reverse lookup of objects is probably what you want:

class person:   
    people_database = {} 
    def __init__(self, name, gender):
        person.people_database[name] = self 
                  #add this instance to the database
                  #possibly overriding the previous entry with this name
        self.name=name
        self.gender=gender

b=person('bob','male')

print (person.people_database["bob"])
print (b)
print(b is person.people_database["bob"])

although do know that if you change the name of a person like:

b=person('bob','male')
b.name = "susan"

this technique will obviously not work properly.