[ Python 常見問題 ] Converting a number to binary with a fixed length

Source From Here
Question
Say I get a random number between 1 and 127. I change the number to binary and remove the 0b from it with the fallowing code:

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1. key_one= int(raw_input("Enter key (0 <= key <= 127): "))  
2.   
3. if key_one in range(128):  
4.     bin_key_one=bin(key_one)[2:]  
5. print bin_key_one  
6. else:  
7.     print "You have to enter key (0 <= key <= 127)"  

Now I want to make it 7-characters long by padding the beginning with zeros as necessary. I think I need to use a for loop, but can someone show me how to do it?

How-To
You can leverage the String Formatting. e.g.

>>> key_one = 4
>>> bin_key_one=bin(key_one )[2:] // Remove the prefix '0b'
>>> bin_key_one
'100'
>>> bin(key_one)
'0b100'
>>> '{0:07b}'.format(key_one) // Print the first input argument as binary string with fixed length=7
'0000100'