Converting to and from Python lists in NumPy - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of converting numpy array to list

What is the output of this code snippet converting a numpy array to a Python list?

NumPy

import numpy as np
arr = np.array([[1, 2], [3, 4]])
result = arr.tolist()
print(result)

A [1, [2, 3], 4]

B [1, 2, 3, 4]

C [[1, 2], [3, 4]]

D [[1, 2, 3, 4]]

💡 Hint

Think about how a 2D numpy array converts to nested lists.

The tolist() method converts a numpy array into a nested Python list preserving the shape. Here, a 2x2 array becomes a list of two lists, each with two elements.

❓ data_output

intermediate

Result of creating numpy array from list

What is the shape of the numpy array created from this list?

NumPy

import numpy as np
lst = [[1, 2, 3], [4, 5, 6]]
arr = np.array(lst)
print(arr.shape)

A (3, 2)

B (2, 3)

C (6,)

D (2,)

💡 Hint

Check how many rows and columns the list has.

The list has 2 sublists each with 3 elements, so the numpy array shape is (2, 3).

🔧 Debug

advanced

Identify the error when converting list to numpy array

What error does this code raise when converting a list with mixed types to a numpy array?

NumPy

import numpy as np
lst = [1, 'two', 3]
arr = np.array(lst)
print(arr)

A SyntaxError: invalid syntax

B TypeError: unsupported operand type(s) for +: 'int' and 'str'

C ValueError: could not convert string to float: 'two'

D No error, output: array(['1', 'two', '3'], dtype='<U21')

💡 Hint

Numpy tries to find a common data type for all elements.

Numpy converts all elements to strings to create a uniform array of strings, so no error occurs.

🚀 Application

advanced

Convert nested list to 1D numpy array

Which option correctly converts a nested list into a flat 1D numpy array?

NumPy

import numpy as np
nested_list = [[1, 2], [3, 4], [5, 6]]

A np.array(nested_list).flatten()

B np.array(nested_list).tolist()

C np.array(nested_list).reshape(2, 3)

D np.array(nested_list).ravel().tolist()

💡 Hint

Flattening means making all elements in one dimension.

The flatten() method returns a copy of the array collapsed into one dimension.

🧠 Conceptual

expert

Understanding difference between tolist() and list() on numpy arrays

What is the key difference between using tolist() and list() on a numpy array?

NumPy

import numpy as np
arr = np.array([[1, 2], [3, 4]])

A tolist() converts the entire array into nested Python lists; list() converts only the top-level array into a list of numpy arrays

B tolist() and list() produce identical outputs

C tolist() flattens the array; list() preserves the shape

D tolist() returns a numpy array; list() returns a Python list

💡 Hint

Think about how each method handles nested structures.

tolist() recursively converts all elements to Python lists, preserving nested structure. list() converts only the outer array to a list, keeping inner elements as numpy arrays.