find first bad version

Question

Working on below find first bad version problem, post my code in Python 2.7, any smarter ideas for better time complexity, any code bug or code style advice are highly appreciated. My major idea is divide and conquer like a binary search.

Problem,

You are a product manager and currently leading a team to develop a new product. Unfortunately, the latest version of your product fails the quality check. Since each version is developed based on the previous version, all the versions after a bad version are also bad. Suppose you have n versions [1, 2, ..., n] and you want to find out the first bad one, which causes all the following ones to be bad. You are given an API bool isBadVersion(version) which will return whether version is bad. Implement a function to find the first bad version. You should minimize the number of calls to the API.

Source code,

def isBadVersion(number):
    if number == 0:
        return False
    else:
        return True
# 0 means good, 1 means bad
def find_first_bad_index(numbers):
    start = 0
    end = len(numbers)-1
    if isBadVersion(numbers[start]) == True:
        return start
    if isBadVersion(numbers[end]) == False:
        raise Exception('all good')
    while start <= end:
        mid = start + (end-start)/2
        if isBadVersion(numbers[mid]) == True:
            if start == end:
                return end
            else:
                end = mid
        else:
            start = mid + 1

if __name__ == "__main__":
    print find_first_bad_index([0,0,0,1,1,1,1,1])
    print find_first_bad_index([0,0,0,0,0,0,0,1])

Answer 1

Obligatory PEP 8 reminder:

• Whitespace is important.
• Whitespace is important too.

• Recommendations state that you should:

  • Design exception hierarchies based on the distinctions that code catching the exceptions is likely to need, rather than the locations where the exceptions are raised. Aim to answer the question "What went wrong?" programmatically, rather than only stating that "A problem occurred" (see PEP 3151 for an example of this lesson being learned for the builtin exception hierarchy);

  • Not compare boolean values to True or False using == .

    Yes:   if greeting:
    No:    if greeting == True:
    Worse: if greeting is True:
    

Applying that to your code gives the more readable:

class NoBadVersion(Exception):
    pass


def isBadVersion(number):
    """Mocked external API. Obligatory camelCaseName for consistency."""
    return number > 0


def find_bad_version(numbers):
    """Return the index of the first bad version.
    Tries to perform as few external API calls as possible.
    """

    start = 0
    end = len(numbers) - 1

    if isBadVersion(numbers[start]):
        return start

    if not isBadVersion(numbers[end]):
        raise NoBadVersion('all good')

    while start <= end:
        mid = start + (end-start) / 2
        if isBadVersion(numbers[mid]):
            if start == end:
                return end
            end = mid
        else:
            start = mid + 1


if __name__ == "__main__":
    print find_bad_version([0, 0, 0, 1, 1, 1, 1, 1])
    print find_bad_version([0, 0, 0, 0, 0, 0, 0, 1])