Bisect Left in Python
Python’s bisect_left
function is implemented as a binary search algorithm. You could certainly implement your own version of bisect_left
to understand how it works under the hood. Here’s a simple Python implementation:


In this function:
arr
is a sorted list.target
is the element you’re searching for. The function returns the index where the
target
should be inserted to keep the list sorted.
You can test this function like so:


The output is 2 because if you insert 3 at index 2 in the list [1, 2, 4, 5]
, the list remains sorted.
Difference
The bisect_left
function and the classic binary search algorithm are very similar, as they both rely on the concept of binary search to perform operations in a sorted list. However, there’s a fundamental difference in their purpose and behavior.
Classic Binary Search:
 Classic binary search is used to find if a particular value exists in a sorted list. If the value exists, the algorithm will return the index of that value. If the value does not exist, it typically returns a value to indicate that the search value was not found in the list.
bisect_left
function:
 The
bisect_left
function in Python is used to find the insertion point for a value in a sorted list to maintain the sorted order of the list. It doesn’t check whether the value exists in the list. Instead, it returns the position where the value should be inserted. If the value is already present in the list,bisect_left
will return the position of the first occurrence of that value.
So, in summary, the primary difference is in the intent and the behavior of these two operations: the classic binary search is for finding a value, whereas bisect_left
is for finding the correct insertion point for a value in a sorted list.