How can I replace named vars in a string in python from an incomplete dictionary?

Question

set defaults

my_data_dict = {'first_name':'','last_name':'','blah':'','something_else':''} #defaults with all values from format string
my_data_dict.update(form) #replace any values we got
 msg = """<html><p>%(Frist_Name)s</p><p>%(Last_Name)s</p></html> """ % my_data_dict

or use default dict (this way you wont need to input all your defaults they will just magically be '')

from collections import defaultdict
my_dict = defaultdict(str)
print my_dict['some_key_that_doesnt_exist']  #should print empty string
my_dict.update(form)
msg = """<html><p>%(Frist_Name)s</p><p>%(Last_Name)s</p></html> """ % my_data_dict

or as abarnert points out you can simplify it to

from collections import defaultdict
my_dict = defaultdict(str,form)
msg = """<html><p>%(Frist_Name)s</p><p>%(Last_Name)s</p></html> """ % my_dict

here is a complete example I just did in my terminal

>>> d = defaultdict(str,{'fname':'bob','lname':'smith','zipcode':11111})
>>> format_str = "Name: %(fname)s  %(lname)s\nPhone: %(telephone)s\nZipcode: %(z
ipcode)s"
>>> d
defaultdict(<type 'str'>, {'lname': 'smith', 'zipcode': 11111, 'fname': 'bob'})
>>> #notice no telephone here
...
>>> d['extra_unneeded_argument'] =' just for show'
>>> d
defaultdict(<type 'str'>, {'lname': 'smith', 'extra_unneeded_argument': ' just f
or show', 'zipcode': 11111, 'fname': 'bob'})
>>> print format_str%d
Name: bob  smith
Phone:
Zipcode: 11111