Converting a decimal integer (denary, base 10) to a binary string (base 2) is amazingly simple. An exception is raised for negative numbers and zero is a special case. To test the result, the binary string is converted back to the decimal value, easily done with the int(bStr, 2) function.

```
# convert a decimal (denary, base 10) integer to a binary string (base 2)
# tested with Python24 vegaseat 6/1/2005
def Denary2Binary(n):
'''convert denary integer n to binary string bStr'''
bStr = ''
if n < 0: raise ValueError, "must be a positive integer"
if n == 0: return '0'
while n > 0:
bStr = str(n % 2) + bStr
n = n >> 1
return bStr
def int2bin(n, count=24):
"""returns the binary of integer n, using count number of digits"""
return "".join([str((n >> y) & 1) for y in range(count-1, -1, -1)])
# this test runs when used as a standalone program, but not as an imported module
# let's say you save this module as den2bin.py and use it in another program
# when you import den2bin the __name__ namespace would now be den2bin and the
# test would be ignored
if __name__ == '__main__':
print Denary2Binary(255) # 11111111
# convert back to test it
print int(Denary2Binary(255), 2) # 255
print
# this version formats the binary
print int2bin(255, 12) # 000011111111
# test it
print int("000011111111", 2) # 255
print
# check the exceptions
print Denary2Binary(0)
print Denary2Binary(-5) # should give a ValueError
```

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