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If Python raises ModuleNotFoundError after you install a package, first make sure the install command and the failing program use the same Python interpreter. Run python -m pip with the interpreter that launches your code, then check the active environment, the package’s documented import name, and—if needed—Python’s module search path.
1. Check which Python runs your code
In the same shell or environment you use to start the failing program, check the interpreter and ask its pip whether the distribution is installed:
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python -c "import sys; print(sys.executable)"
python -m pip --version
python -m pip show package-nameReplace package-name with the distribution name used in the install command. If python is not the command that launches your script, use the matching executable instead. For example, use a version-qualified command such as python3.12 where appropriate, or the matching py -3.x launcher on Windows. The Python Packaging User Guide documents these forms for installations with multiple Python versions.
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python -m pip install package-name2. Make installation and execution use the same environment
A virtual environment has its own interpreter context and package locations. Activate the intended environment before installing and running the program, or call that environment’s interpreter directly. Python’s path initialization documentation describes how pyvenv.cfg determines the virtual-environment prefix, and the Packaging User Guide covers installing packages in an active environment.
A terminal install does not establish that a notebook kernel, IDE, service, or scheduled job uses that same environment. From the failing process, print sys.executable and compare it with the executable used for installation. If they differ, select the intended interpreter or kernel, then install the package through that interpreter.
3. Confirm the name used by the import
The name supplied to pip identifies a distribution; an import statement refers to a Python module or package provided by that distribution. Those names are not guaranteed to match. Check the package’s official usage instructions for the exact import statement rather than guessing based on spelling, punctuation, capitalization, or singular and plural forms.
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4. Inspect the failing process’s search path
If the interpreter and environment appear correct, inspect these values inside the process that raises the exception:
import sys
print(sys.executable)
print(sys.prefix)
print(sys.path)sys.path is the module search path Python uses to locate imports. The path initialization documentation explains that Python initializes this path at startup; the site documentation describes how site processing adds package locations and how environment prefixes and site configuration can affect them.
If the expected package location is missing, look for a mismatch in the process’s interpreter or startup configuration. Avoid adding a permanent PYTHONPATH as a first resort: Python’s documentation notes that it affects all installed Python versions and environments, not just the one you are troubleshooting.
5. Distinguish a missing module from another failure
- The install command reported an error: read its output first. A command that failed does not show that the package was installed.
pip showcannot find the distribution: install with the interpreter that runs the program, usingpython -m pip install package-nameor its matching version-qualified equivalent.- The import name may be different: check the package’s official usage instructions for the exact module name.
- Import starts but another exception appears: diagnose that traceback as a separate runtime or package-specific problem instead of repeatedly reinstalling.
Python and pip documentation are versioned, and path behavior can vary by Python release. Use the documentation for the interpreter version and environment in question; the commands above identify the relevant executable but cannot diagnose a machine-specific configuration by themselves.
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