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How to Solve Python TypeError: ‘map’ object is not subscriptable

by | Programming, Python, Tips

In Python 3, a map object is an iterator and is not subscriptable. If you try to access items inside a map object using the subscript operator [], you will raise the TypeError: ‘map’ object is not subscriptable.

This error typically occurs when using Python 2 syntax when using Python 3. In Python 2, calling the built-in map() function returns a list, which is subscriptable.

You can solve this error by converting the map object to a list using the built-in list function. For example,

new_list = list(map(int, old_list))

You can also iterate over the values in the iterator using a for loop

This tutorial will go through the error in detail and how to solve it with code examples.


TypeError: ‘map’ object is not subscriptable

A TypeError occurs when you perform an illegal operation for a specific data type.

What Does Subscriptable Mean?

The subscript operator, which is square brackets [], retrieves items from subscriptable objects like lists or tuples. The operator calls the __getitem__ method, for example, a[i] is equivalent to a.__getitem__(i).

All subscriptable objects have a __getitem__ method. Map objects are iterators and do not have a __getitem__ method. We can verify that map objects do not have the __getitem__ method by defining a creating a map object and passing it to the dir() method:

def square(i):
    res = i ** 2
    return res

lst = [2, 3, 4, 5]

squared = map(square, lst)

print(dir(squared))
['__class__', '__delattr__', '__dir__', '__doc__', '__eq__', '__format__', '__ge__', '__getattribute__', '__gt__', '__hash__', '__init__', '__init_subclass__', '__iter__', '__le__', '__lt__', '__ne__', '__new__', '__next__', '__reduce__', '__reduce_ex__', '__repr__', '__setattr__', '__sizeof__', '__str__', '__subclasshook__']

 A Python iterator object must implement two special methods, __iter__() and __next__(), collectively called the iterator protocol.

Example

This error typically occurs when using Python 2 map operations in Python 3. First, let’s verify what the built-in map function returns in Python 2. We will use a virtual environment with Python 2 installed and confirm that we are using Python 2 using the sys library:

import sys

print(sys.version)
2.7.16 |Anaconda, Inc.| (default, Sep 24 2019, 16:55:38) 
[GCC 4.2.1 Compatible Clang 4.0.1 (tags/RELEASE_401/final)]

Next, we will define a list of strings, which we will convert to a list of integers using the map() function. We will pass the object returned by the map function to the type() function to get the type.

string_list = ["2", "3", "4", "5", "6"]

int_list = map(int, string_list)

print(type(int_list))

Let’s run the code to get the result:

<type 'list'>

In Python 2, the map function returns a list, which is subscriptable. We access an element of the list using the subscript operator, for example:

print(int_list[0])
2

Let’s try to do this using Python 3. We will use a different virtual environment with Python 3 installed, which we will verify using sys.

import sys

print(sys.version)
3.8.12 (default, Oct 12 2021, 06:23:56) 
[Clang 10.0.0 ]

Next, we will define our list of strings and use the map() function to convert it to a list of integers. We will pass the object returned by the map function to the type function.

string_list = ["2", "3", "4", "5", "6"]

int_list = map(int, string_list)

print(type(int_list))

Let’s run the code to see the result:

<class 'map'>

In Python 3, the map function returns a map object, which is an iterator. If we try to access an element of the map object using the subscript operator, we will raise the TypeError.

print(int_list[0])
---------------------------------------------------------------------------
TypeError                                 Traceback (most recent call last)
Input In [4], in <cell line: 1>()
----> 1 print(int_list[0])

TypeError: 'map' object is not subscriptable

Solution #1

We can convert map objects to lists using the built-in list() function. Let’s look at the revised code:

string_list = ["2", "3", "4", "5", "6"]

int_list = list(map(int, string_list))

print(type(int_list))

print(int_list[0])

Let’s run the code to get the result:

<class 'list'>
2

Solution #2

We can access values in an iterator using a for loop, which invokes the __next__() method of the map iterator. Let’s look at the revised code:

string_list = ["2", "3", "4", "5", "6"]
int_list = map(int, string_list)

for i in int_list:
    print(i)

Let’s run the code to get the result:

2
3
4
5
6

Summary

Congratulations on reading to the end of this tutorial! The TypeError: ‘map’ object is not subscriptable occurs when you try to access an item from a map object in Python 3. The Python 3 map object is an iterator and is not subscriptable. We can convert map objects to lists which are subscriptable.

For further reading on TypeErrors, go to the article: How to Solve Python TypeError: ‘function’ object is not subscriptable

Go to the online courses page on Python to learn more about Python for data science and machine learning.

Have fun and happy researching!

Research Scientist at Moogsoft | + posts

Suf is a research scientist at Moogsoft, specializing in Natural Language Processing and Complex Networks. Previously he was a Postdoctoral Research Fellow in Data Science working on adaptations of cutting-edge physics analysis techniques to data-intensive problems in industry. In another life, he was an experimental particle physicist working on the ATLAS Experiment of the Large Hadron Collider. His passion is to share his experience as an academic moving into industry while continuing to pursue research. Find out more about the creator of the Research Scientist Pod here and sign up to the mailing list here!