Seeing this weirdness:

statler:[~/src/pants][fix_urls]$ RUST_BACKTRACE=1 ./pants
10:56:30.54 [INFO] initializing scheduler...
10:56:30.98 [ERROR] panic at 'called `Result::unwrap()` on an `Err` value: PyErr { ptype: <class 'AttributeError'>, pvalue: Some("module 'pants.util.docutil' has no attribute 'docs_url'"), ptraceback: None }', src/externs/mod.rs:265
10:56:30.98 [ERROR] Please set RUST_BACKTRACE=1, re-run, and then file a bug at <https://github.com/pantsbuild/pants/issues>.
10:56:30.98 [INFO] Filesystem changed during run: retrying `ResolvedPluginDistributions` in 500ms...
10:56:31.49 [ERROR] panic at 'called `Result::unwrap()` on an `Err` value: PyErr { ptype: <class 'AttributeError'>, pvalue: Some("module 'pants.util.docutil' has no attribute 'docs_url'"), ptraceback: None }', src/externs/mod.rs:265
10:56:31.49 [ERROR] Please set RUST_BACKTRACE=1, re-run, and then file a bug at <https://github.com/pantsbuild/pants/issues>.
10:56:31.49 [INFO] Filesystem changed during run: retrying `ResolvedPluginDistributions` in 500ms...
10:56:31.99 [ERROR] panic at 'called `Result::unwrap()` on an `Err` value: PyErr { ptype: <class 'AttributeError'>, pvalue: Some("module 'pants.util.docutil' has no attribute 'docs_url'"), ptraceback: None }', src/externs/mod.rs:265
10:56:31.99 [ERROR] Please set RUST_BACKTRACE=1, re-run, and then file a bug at <https://github.com/pantsbuild/pants/issues>.
10:56:32.00 [INFO] Filesystem changed during run: retrying `ResolvedPluginDistributions` in 500ms...
10:56:32.50 [ERROR] panic at 'called `Result::unwrap()` on an `Err` value: PyErr { ptype: <class 'AttributeError'>, pvalue: Some("module 'pants.util.docutil' has no attribute 'docs_url'"), ptraceback: None }', src/externs/mod.rs:265
...

Force kill the process and try again. It's probably from modifying that Python function , as it's used by Rust via FFI. I've noticed that recently that a rust error like that hangs the Python process

I should have mentioned that this happens after killing pantsd