An ImportError occurs in Python when an import statement fails to find or load a module. The most common subclass is ModuleNotFoundError (Python 3.6+), which specifically means the module does not exist in the current environment. This error typically happens when a package is not installed, the module name is misspelled, or the Python environment is incorrect.
# ❌ Problem
import requests # ImportError: No module named 'requests'
# ✅ Solution: Install the package first
# pip install requests
# Then import works:
import requests
response = requests.get("https://api.example.com/data")Third-party packages like requests, numpy, or pandas are not part of Python's standard library. You must install them with pip before importing them.
Module names are case-sensitive and must be spelled exactly. The package name you install with pip is sometimes different from the module name you import (e.g., pip install Pillow but import PIL).
A circular import occurs when module A imports module B, and module B also imports module A. Python partially loads modules, so the second import may fail to find names that haven't been defined yet.
When using virtual environments, a package installed in one environment is not available in another. This is a common issue when switching between projects or when an IDE uses a different Python interpreter than the terminal.
import numpy as np # ModuleNotFoundError: No module named 'numpy'
import pandas as pd # ModuleNotFoundError: No module named 'pandas'# ✅ Install via pip in your terminal
# pip install numpy pandas
# ✅ Or install from requirements.txt
# pip install -r requirements.txt
# ✅ Verify installation
# pip show numpy
import numpy as np # Works after installation
import pandas as pd # Works after installationimport Requests # ModuleNotFoundError (should be lowercase 'requests')
import sklearn # ModuleNotFoundError (pip install scikit-learn, import sklearn)
import cv2 # ModuleNotFoundError (pip install opencv-python, import cv2)import requests # ✅ lowercase
# pip install scikit-learn
import sklearn # ✅ import name differs from pip name
# pip install opencv-python
import cv2 # ✅ import name differs from pip name
# pip install Pillow
from PIL import Image # ✅ import PIL, not Pillow# module_a.py
from module_b import func_b # ImportError: cannot import name 'func_b'
def func_a():
return "A"
# module_b.py
from module_a import func_a # Circular import!
def func_b():
return func_a() + "B"# ✅ Solution 1: Move shared code to a third module (module_common.py)
# module_common.py
def func_a():
return "A"
# module_b.py
from module_common import func_a
def func_b():
return func_a() + "B"
# ✅ Solution 2: Import inside the function (lazy import)
# module_a.py
def func_a():
from module_b import func_b # Import only when needed
return func_b()# Installed requests in venv1, but running script with venv2 or system Python
# pip install requests (in venv1)
# python script.py (using system Python "” no requests!)
import requests # ModuleNotFoundError# ✅ Activate the correct virtual environment first
# Windows: venv\Scripts\activate
# macOS/Linux: source venv/bin/activate
# ✅ Verify which Python you're using
import sys
print(sys.executable) # Shows the Python interpreter path
# ✅ Install in the active environment
# python -m pip install requests (uses the active Python's pip)ModuleNotFoundError is a subclass of ImportError introduced in Python 3.6. ImportError is the broader error for any import failure (e.g., importing a name that does not exist in a module). ModuleNotFoundError specifically means the module itself was not found.
Run pip install package-name in your terminal. Make sure your virtual environment is activated first. Use python -m pip install package-name to ensure you install into the correct Python environment.
You likely installed the package in a different Python environment than the one running your script. Check which Python is active with python --version and which pip is used with pip --version. They should point to the same environment.
Wrap the import in try/except ImportError: try: import numpy as np; except ImportError: np = None. Then check if np is None before using it.
This means the module exists but does not have the name you are trying to import. Either the name is misspelled, it was removed in a newer version, or you have a version mismatch. Check the module's documentation for the correct name.
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