Converting to and from Python lists in NumPy - Cheat Sheet & Quick Revision

Recall & Review

beginner

How do you convert a Python list to a NumPy array?

Use numpy.array() function. For example, np.array([1, 2, 3]) converts the list [1, 2, 3] to a NumPy array.

beginner

How do you convert a NumPy array back to a Python list?

Use the tolist() method of the NumPy array. For example, arr.tolist() converts the array arr to a Python list.

beginner

What is the difference between a Python list and a NumPy array?

A Python list can hold items of different types and is flexible but slower. A NumPy array holds items of the same type and is optimized for fast numerical operations.

intermediate

Can you convert a nested Python list to a NumPy array? How?

Yes, you can convert nested lists (lists of lists) to multi-dimensional NumPy arrays using np.array(). For example, np.array([[1, 2], [3, 4]]) creates a 2x2 array.

intermediate

What happens if you convert a NumPy array with mixed data types to a list?

The tolist() method converts the array elements to native Python types, preserving the structure. If the array has mixed types, the list will contain those types as Python objects.

Which function converts a Python list to a NumPy array?

A array.tolist()

B numpy.tolist()

C numpy.array()

D list()

The numpy.array() function converts a Python list to a NumPy array.

How do you convert a NumPy array named arr to a Python list?

A arr.tolist()

B arr.to_list()

C list(arr)

D numpy.list(arr)

Use the tolist() method: arr.tolist().

What type of data structure does np.array() create from a list?

A NumPy array

B Python list

C Dictionary

D Tuple

np.array() creates a NumPy array.

Can np.array() convert a nested list into a multi-dimensional array?

A Only for 1D lists

B No

C Only if the nested lists are the same length

D Yes

Yes, np.array() can convert nested lists into multi-dimensional arrays.

What does arr.tolist() return?

A A NumPy array

B A Python list

C A Python tuple

D A Python dictionary

arr.tolist() returns a Python list.

Explain how to convert a Python list to a NumPy array and back to a list.

Think about the functions and methods used for conversion.

Describe the difference between Python lists and NumPy arrays in terms of data type and performance.

Consider flexibility vs speed.

Practice

1. What does the function np.array() do when given a Python list?

easy

A. It deletes the Python list.

B. It converts the Python list into a Python dictionary.

C. It converts the Python list into a NumPy array.

D. It converts the Python list into a string.

Solution

1. Step 1: Understand np.array() purpose

   The function np.array() is designed to take a Python list and create a NumPy array from it.

2. Step 2: Compare options with function behavior

   Only It converts the Python list into a NumPy array. correctly states that it converts a list to a NumPy array. Other options describe unrelated actions.

3. Final Answer:

   It converts the Python list into a NumPy array. -> Option C

4. Quick Check:

   np.array(list) = NumPy array [OK]

Hint: np.array() always makes a NumPy array from a list [OK]

Common Mistakes:

• Thinking np.array() converts to dict or string
• Confusing np.array() with list methods
• Assuming np.array() deletes data

2. Which of the following is the correct syntax to convert a NumPy array arr back to a Python list?

easy

A. arr.tolist()

B. arr.to_list()

C. tolist(arr)

D. list(arr.to)

Solution

1. Step 1: Recall the method to convert NumPy array to list

   The correct method is tolist() called on the array object.

2. Step 2: Check each option's syntax

   arr.tolist() uses arr.tolist(), which is correct. arr.to_list() has a wrong method name. Options C and D are invalid syntax.

3. Final Answer:

   arr.tolist() -> Option A

4. Quick Check:

   array.tolist() converts to list [OK]

Hint: Use arr.tolist() to get a Python list from NumPy array [OK]

Common Mistakes:

• Using arr.to_list() instead of arr.tolist()
• Trying to call tolist() as a standalone function
• Incorrect method chaining or syntax errors

3. What is the output of this code?

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

medium

A. [[1], [2], [3]]

B. array([1, 2, 3])

C. TypeError

D. [1, 2, 3]

Solution

1. Step 1: Convert list to NumPy array

   The list [1, 2, 3] is converted to a NumPy array arr with values [1 2 3].

2. Step 2: Convert NumPy array back to list

   Calling arr.tolist() converts the array back to a Python list [1, 2, 3].

3. Final Answer:

   [1, 2, 3] -> Option D

4. Quick Check:

   arr.tolist() = original list [OK]

Hint: arr.tolist() returns the original list values [OK]

Common Mistakes:

• Expecting array output instead of list
• Confusing print output with array representation
• Thinking it returns nested lists

4. Identify the error in this code snippet:

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

medium

A. arr.to_list() is not a valid method

B. np.array() cannot convert lists

C. lst should be a NumPy array already

D. Missing import statement

Solution

1. Step 1: Check method names on NumPy arrays

   The method to convert a NumPy array to a list is tolist(), not to_list().

2. Step 2: Identify the error in the code

   Using arr.to_list() causes an AttributeError because the method does not exist.

3. Final Answer:

   arr.to_list() is not a valid method -> Option A

4. Quick Check:

   Use arr.tolist(), not arr.to_list() [OK]

Hint: Use arr.tolist(), not arr.to_list() [OK]

Common Mistakes:

• Typing to_list() instead of tolist()
• Assuming np.array() fails on lists
• Forgetting to import numpy

5. You have a nested Python list nested = [[1, 2], [3, 4]]. Which code correctly converts it to a NumPy array and then back to a nested Python list?

hard

A. arr = np.array(nested)
new_list = list(arr)

B. arr = np.array(nested)
new_list = arr.tolist()

C. arr = list(nested)
new_list = arr.tolist()

D. arr = np.array(nested)
new_list = arr.to_list()

Solution

1. Step 1: Convert nested list to NumPy array

   Using np.array(nested) converts the nested list into a 2D NumPy array.

2. Step 2: Convert NumPy array back to nested list

   Calling arr.tolist() converts the 2D array back to the original nested Python list structure.

3. Final Answer:

   arr = np.array(nested) new_list = arr.tolist() -> Option B

4. Quick Check:

   np.array() + tolist() preserves nested lists [OK]

Hint: Use np.array() then tolist() for nested list conversion [OK]

Common Mistakes:

• Using list(arr) which flattens the array
• Calling arr.to_list() which is invalid
• Trying to convert list before np.array()