IsADirectoryError: [Errno 21] Is a directory

The IsADirectoryError: [Errno 21] Is a directory occurs in Python when you try to perform file-specific operations (like reading or writing) on a directory instead of a file.

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1. Common Causes and Fixes

Cause 1: Trying to Open a Directory as a File

If you attempt to open a directory instead of a file, Python will raise this error.

Incorrect Code:

with open("my_folder", "r") as file:
    content = file.read()

Problem: "my_folder" is a directory, not a file.

Solution: Check if the path is a directory before trying to open it.

import os

path = "my_folder"
if os.path.isdir(path):
    print("This is a directory, not a file!")
else:
    with open(path, "r") as file:
        content = file.read()

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Cause 2: Trying to Write to a Directory

If you try to write to a directory instead of a file, Python will throw an error.

Incorrect Code:

with open("documents", "w") as file:
    file.write("Hello, world!")

Problem: "documents" is a folder, not a file.

Fix: Ensure the path points to a valid file, not a directory.

file_path = "documents/output.txt"  # Ensure you're writing to a file
with open(file_path, "w") as file:
    file.write("Hello, world!")

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Cause 3: Passing a Directory Instead of a File Path in a Function

Some functions expect a file path, but if you mistakenly pass a directory, Python will throw this error.

Incorrect Code:

import pandas as pd

df = pd.read_csv("my_folder")  # Incorrect: "my_folder" is a directory

Problem: read_csv() expects a CSV file, not a directory.

Solution: Pass the correct file path:

df = pd.read_csv("my_folder/data.csv")  # Ensure a valid CSV file

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Cause 4: Using os.remove() or os.unlink() on a Directory

os.remove() and os.unlink() can only delete files, not directories.

Incorrect Code:

import os

os.remove("my_folder")  # Trying to remove a directory

Problem: os.remove() only works for files, not folders.

Fix: Use os.rmdir() (only if the folder is empty) or shutil.rmtree() (to delete non-empty directories):

import shutil

shutil.rmtree("my_folder")  # Removes directory and contents

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Cause 5: Using open() on an Incorrect File Path

Sometimes, an incorrect file path might point to a directory instead of a file.

Incorrect Code:

file_path = "/home/user/documents"  # This is a directory
with open(file_path, "r") as file:
    content = file.read()

Problem: "documents" is a directory, not a file.

Fix: Append the actual file name to the path.

file_path = "/home/user/documents/report.txt"  # Now it's a file
with open(file_path, "r") as file:
    content = file.read()

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Cause 6: Using shutil.copy() or shutil.move() with a Directory Path

shutil.copy() and shutil.move() expect a file path, not a directory.

Incorrect Code:

import shutil

shutil.copy("my_folder", "backup_folder")  # This will cause an error

Problem: "my_folder" is a directory.

Solution: Use shutil.copytree() for directories:

shutil.copytree("my_folder", "backup_folder")

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2. Handling “IsADirectoryError” in Python

Check if the Path is a File Before Accessing

import os

path = "my_folder"

if os.path.isfile(path):  # Ensure it's a file
    with open(path, "r") as file:
        content = file.read()
else:
    print("Error: This is a directory, not a file!")

Use Try-Except to Handle the Error Gracefully

try:
    with open("my_folder", "r") as file:
        content = file.read()
except IsADirectoryError:
    print("Cannot open a directory as a file. Check your file path.")

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3. Summary of Fixes

Issue	Fix
Opening a directory as a file	Check with os.path.isdir() before opening
Writing to a directory instead of a file	Ensure path points to a file, not a folder
Passing a directory to read_csv() or open()	Provide the correct file path
Using os.remove() on a folder	Use shutil.rmtree() instead
Copying a directory using shutil.copy()	Use shutil.copytree() instead