[Python] Why does tkinter (or turtle) seem to be missing or broken? Shouldn't it be part of...

I have seen many different things go wrong when trying to use the Tkinter standard library package, or its related functionality (turtle graphics using turtle and the built-in IDLE IDE), or with third-party libraries that have this as a dependency (e.g. displaying graphical windows with Matplotlib).

It seems that even when there isn't a problem caused by shadowing the name of the standard library modules (this is a common problem for beginners trying to follow a tutorial and use turtle graphics - example 1; example 2; example 3; example 4), it commonly happens that the standard library Tkinter just doesn't work. This is a big problem because, again, a lot of beginners try to follow tutorials that use turtle graphics and blindly assume that the turtle standard library will be present.

The error might be reported:

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  As ModuleNotFoundError: No module named 'tkinter'; or an ImportError with the same message; or with different casing (I am aware that the name changed from Tkinter in 2.x to tkinter in 3.x; that is a different problem).
  
  
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  Similarly, but referring to an internal _tkinter module, and displaying code with a comment that says "If this fails your Python may not be configured for Tk"; or with a custom error message that says "please install the python-tk package" or similar.
  
  
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  As "No module named turtle" when trying to use turtle specifically, or one of the above errors.
  
  
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  When trying to display a plot using Matplotlib; commonly, this will happen after trying to change the backend, which was set by default to avoid trying to use Tkinter.
  

Why do problems like this occur, when Tkinter is documented as being part of the standard library? How can I add or repair the missing standard library functionality? Are there any special concerns for specific Python environments?


See also: "UserWarning: Matplotlib is currently using agg, which is a non-GUI backend, so cannot show the figure." when plotting figure with pyplot on Pycharm . It is possible to use other GUI backends with Matplotlib to display graphs; but if the TkAgg backend does not work, that is because of a missing or faulty Tkinter install.

In Python 3.x, the name of the Tkinter standard library module was corrected from Tkinter to tkinter (i.e., all lowercase) in order to maintain consistent naming conventions. Please use Difference between tkinter and Tkinter to close duplicate questions caused by trying to use the old name in 3.x (or the new name in 2.x). This question is about cases where Tkinter is not actually available. If it isn't clear which case applies, please either offer both duplicate links, or close the question as "needs debugging details".

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