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How to Solve "NameError: name 'plt' is not defined" and "NameError: name 'matplotlib' is not defined" in Python

The errors NameError: name 'plt' is not defined and NameError: name 'matplotlib' is not defined in Python both indicate the same problem: you're trying to use the matplotlib library (or its pyplot module, commonly aliased as plt) without importing it first.

This guide explains the cause of these errors and provides the correct solutions.

Understanding the NameError​

Both of these NameError messages mean that Python doesn't recognize the name plt or matplotlib. This happens because you haven't told Python where to find these names. In Python, you must explicitly import modules and packages before you can use them. matplotlib is a third-party library for plotting, and matplotlib.pyplot (often imported as plt) is a submodule that provides a convenient interface for creating plots.

Solution: Importing matplotlib.pyplot​

The standard and recommended way to import matplotlib.pyplot is:

import matplotlib.pyplot as plt  # Import and alias as plt

fig, ax = plt.subplots()  # Now you can use plt
print(fig)
print(ax)
  • import matplotlib.pyplot as plt: This line does three things:
    • import matplotlib.pyplot: Imports the pyplot module from the matplotlib package.
    • as plt: Gives it the alias plt. This is a very common convention. Using plt is much shorter and more readable than typing matplotlib.pyplot everywhere.
  • You can also use from matplotlib import pyplot as plt, which imports only the pyplot module.

Common Import Mistakes to Avoid​

Here are some common mistakes that can still lead to a NameError even if you think you've imported matplotlib:

Importing Inside a Function (Local Scope)​

If you import matplotlib inside a function, it's only available within that function's scope:

def example():
    import matplotlib.pyplot as plt  # BAD: Local import

    fig, ax = plt.subplots()
    print(fig)
    print(ax)

# ⛔️ NameError: name 'plt' is not defined (outside the function)
# fig, ax = plt.subplots()

Solution: Always import modules at the top level of your script:

import matplotlib.pyplot as plt  # GOOD: Top-level import

def example():
    fig, ax = plt.subplots()
    print(fig)
    print(ax)

fig, ax = plt.subplots() # Works now

Importing Inside a try-except Block​

Avoid importing inside a try-except block that might fail:

try:
    # Code here could raise an error
    import matplotlib.pyplot as plt
    fig, ax = plt.subplots()
    print(fig)
    print(ax)
except ImportError: # Exception is caught here
    print("matplotlib is not found.")

# ⛔️ NameError: name 'plt' is not defined (if import failed)
#fig, ax = plt.subplots()
  • The plt won't be accessible outside the try/except block.

Solution: Import at the top level, outside any try-except blocks:

import matplotlib.pyplot as plt # Correct import

try:
    fig, ax = plt.subplots()
    print(fig)
    print(ax)
except ImportError: # This is used in case of failure
    print("matplotlib is not found.")

Typos and Case Sensitivity​

Python is case-sensitive. plt, matplotlib, and pyplot must be spelled and capitalized exactly as shown. Double-check for typos.

Shadowing the module​

Don't name your files or variables with the same name as the library, otherwise the import will not work as expected.

Conclusion​

The NameError: name 'plt' is not defined and NameError: name 'matplotlib' is not defined errors are easily fixed by correctly importing the matplotlib.pyplot module (usually as plt) at the top of your Python file.

Avoid importing inside functions or try-except blocks.

By following these best practices, you can ensure that matplotlib is available throughout your code. If you have the library installed and you are still getting this error, make sure you are using the correct Python interpreter (and virtual environment, if applicable).

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