TL;DR: your xgboost is ancient; the sklearn-style XGBClassifier was added later. Upgrade xgboost and the import works.

## The error
```text
ImportError: cannot import name 'XGBClassifier' from 'xgboost'
```

## Fix it
1. Upgrade xgboost:
   ```
   pip install -U xgboost
   ```
   Expected: installs a modern release.
2. Verify:
   ```
   python -c "from xgboost import XGBClassifier; print('ok')"
   ```
   Expected: prints ok.

## When this applies
Use this when code does `from xgboost import XGBClassifier` on a very old xgboost.

## When it does not apply
If xgboost is already modern and the import fails, the install is corrupt; reinstall. If the error is the native-library load failure instead, that is a separate missing-.so problem.

## Compatibility
xgboost 0.7+. XGBClassifier has existed for years; only truly ancient installs lack it.

## Root cause
Early xgboost exposed only the low-level xgb.train API. The sklearn-compatible wrappers came later, so installs from that era have no XGBClassifier name to import.

## Edge cases
Upgrading across many major versions can change default parameters; validate model metrics after the jump. If a project pins the old xgboost deliberately, the sklearn API simply is not available there.