Interview Q&A / Python

1. How do dictionary and set comprehensions work in Python?

   A dict comprehension uses curly braces with key: value, as in {k: v for ...}. A set comprehension uses curly braces with one expression, as in {x for ...}, and removes duplicates automatically.

   BeginnerQuick answer1 min read
2. In a list comprehension, where does the if go: before or after the for?

   A filter if goes after the for clause and drops items. An if-else that chooses between two values is an expression, so it goes before the for. Writing else after the for is a SyntaxError.

   BeginnerQuick answer1 min read
3. What is a list comprehension in Python, and when should you use one?

   A list comprehension builds a list in one expression: [expr for item in iterable if condition]. It is shorter and usually faster than an append loop. Use it for simple transform-and-filter work, not for complicated logic.

   BeginnerQuick answer1 min read
4. Write a function to flatten a list of lists into a single list.

   For one level of nesting, use a nested comprehension: [x for row in rows for x in row]. For arbitrary depth, use recursion and check isinstance(item, list). The flat version is O(n) in the number of elements.

   BeginnerQuick answerCoding1 min read
5. Does the loop variable of a list comprehension leak into the surrounding scope in Python 3?

   No. In Python 3 a comprehension has its own scope, so its loop variable does not leak. A normal for loop does leave its variable behind. In Python 2, list comprehensions did leak.

   IntermediateQuick answer1 min read
6. Write a function to transpose a matrix (swap its rows and columns).

   Use zip(*matrix) and convert each tuple to a list, or use a nested comprehension indexing matrix[r][c]. The first row becomes the first column. The time is O(rows x cols).

   IntermediateQuick answerCoding1 min read