🎨 Delay the YAPF import until it's actually needed.

[coderanger]

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Motivation

This avoids a hard dependency on lib2to3 which is not included in most minimal Python distributions such as in a container environment.

Modification

The import is moved inside the function so yapf is only needed if the pretty-print functionality is actually in use.

BC-breaking (Optional)

No compat effect.

Use cases (Optional)

N/A other than making it easier to use MMCV in minimal environment.

Checklist

Before PR:

• [ ] I have read and followed the workflow indicated in the CONTRIBUTING.md to create this PR.
• [ ] Pre-commit or linting tools indicated in CONTRIBUTING.md are used to fix the potential lint issues.
• [ ] Bug fixes are covered by unit tests, the case that causes the bug should be added in the unit tests.
• [ ] New functionalities are covered by complete unit tests. If not, please add more unit test to ensure the correctness.
• [ ] The documentation has been modified accordingly, including docstring or example tutorials.

After PR:

• [ ] If the modification has potential influence on downstream or other related projects, this PR should be tested with some of those projects, like MMDet or MMCls.
• [ ] CLA has been signed and all committers have signed the CLA in this PR.

[CLAassistant]

All committers have signed the CLA.

[coderanger]

Since FormatCode is optional, maybe we can import it by try... except and throw a not formatted string warning.

Also an option, but having features that vary based on which libraries are installed can lead to unexpected deployment woes. If going that route, I would probably add a format=True/False parameter to the method so the user can request the desired behavior.

[HAOCHENYE]

Since FormatCode is optional, maybe we can import it by try... except and throw a not formatted string warning.

Also an option, but having features that vary based on which libraries are installed can lead to unexpected deployment woes. If going that route, I would probably add a format=True/False parameter to the method so the user can request the desired behavior.

Sorry for my late reply, If format=True/False is a parameter of the method, what's the default behavior of pretty_text, BTW, it is a property actually. In my opinion, the default behavior of xxx.pretty_text should not raise an ImportError whether yapf is installed or not. Consider the case user export the config by cfg.pretty_text at the end of training， the ImportError could break the saving process.

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