Walk Into Your Next AI Interview
Fully Prepared.

What is the difference between a list and a tuple in Python?

Lists are mutable (can be changed after creation) and use square brackets []. Tuples are immutable (cannot be changed once created) and use parentheses (). Because tuples are immutable, they're slightly faster and can be used as dictionary keys, while lists cannot.

Write a Python function to check if a string is a palindrome.

def is_palindrome(s): s = s.lower().replace(' ', '') return s == s[::-1] This compares the string to its reverse using slice notation [::-1]. For very long strings, a two-pointer approach (comparing from both ends inward) is more memory-efficient.

You're given a CSV with 2 million rows and Pandas runs out of memory loading it. What do you do?

Several strategies: (1) Use the dtype parameter to specify smaller data types (e.g. int8 instead of int64). (2) Use chunksize in pd.read_csv() to process in batches. (3) Load only needed columns with usecols. (4) Consider Dask or Polars for out-of-core processing if the file genuinely doesn't fit in memory.

Explain the difference between INNER JOIN and LEFT JOIN in SQL.

INNER JOIN returns only rows with matching values in both tables. LEFT JOIN returns all rows from the left table, with NULLs for right-table columns where there's no match. Use LEFT JOIN when you need to preserve all records from your primary table regardless of matches.

Write a SQL query to find the second-highest salary from an Employees table.

SELECT MAX(salary) FROM Employees WHERE salary < (SELECT MAX(salary) FROM Employees); This finds the maximum salary that is less than the overall maximum — giving the second-highest. For the Nth highest generally, use DENSE_RANK() as a window function instead.

What are *args and **kwargs used for in Python functions?

*args lets a function accept any number of positional arguments, collected as a tuple. **kwargs lets it accept any number of keyword arguments, collected as a dictionary. They're used when you don't know in advance how many arguments will be passed — common in wrapper functions and decorators.

Write a Pandas snippet to find and fill missing values in a column with the column's median.

df['column_name'] = df['column_name'].fillna(df['column_name'].median()) Median is often preferred over mean for filling missing values because it's robust to outliers — a few extreme values won't skew the median the way they would the mean.

What is the difference between == and is in Python?

== checks value equality (do two objects have the same value?). is checks identity equality (are two variables pointing to the exact same object in memory?). Two lists with identical contents will be == but not is, since they're separate objects.

Your manager asks you to find duplicate rows in a 500,000-row SQL table and remove them, keeping only the first occurrence. How do you approach this?

Use ROW_NUMBER() partitioned by the columns that define a duplicate, ordered by an id or timestamp column, then delete rows where the row number is greater than 1: DELETE FROM table_name WHERE id NOT IN ( SELECT id FROM ( SELECT id, ROW_NUMBER() OVER (PARTITION BY col1, col2 ORDER BY id) as rn FROM table_name ) t WHERE rn = 1 );

What is a virtual environment in Python, and why is it important?

A virtual environment is an isolated Python environment with its own installed packages, separate from your system's global Python. It prevents dependency conflicts between projects — e.g. Project A needing pandas 1.5 and Project B needing pandas 2.0 can coexist without breaking each other.