Are python objects thread-safe?

[RajNewbie]

Say, I have two threads, updating the same dictionary object - but for
different parameters:
Please find an example below:
a = {file1Data : '',
       file2Data : ''}

Now, I send it to two different threads, both of which are looping
infinitely:
In thread1:
a['file1Data'] = open(filename1).read
          and
in thread2:
a['file2Data'] = open(filename2).read

My question is  - is this object threadsafe? - since we are working on
two different parameters in the object. Or should I have to block the
whole object?