TL;DR: sympy requires the mpmath package and it is not installed. Run `pip install mpmath` and sympy imports fine.

## The error
```text
ModuleNotFoundError: No module named 'mpmath'
```

## Fix it
1. Install mpmath:
   ```
   pip install mpmath
   ```
   Expected: install completes.
2. Verify:
   ```
   python -c "import sympy; print(sympy.__version__)"
   ```
   Expected: prints the version, no error.

## When this applies
Use this when `import sympy` raises the mpmath error, common in minimal installs and browser-based environments that assemble dependencies by hand.

## When it does not apply
If mpmath is installed but sympy still fails, check you are in the right environment. If the error names a different module, install that one instead.

## Compatibility
All sympy versions; mpmath has always been the required math backend.

## Root cause
sympy does arbitrary-precision arithmetic through mpmath rather than reimplementing it. The dependency is declared, but hand-assembled or --no-deps installs can leave it out.

## Edge cases
Some packagers split sympy into subpackages; make sure the mpmath requirement of the exact sympy build is satisfied. In locked environments, add mpmath next to sympy in the requirements file.