After upgrading to Python 3.6, I'm working towards cleaning up "DeprecationWarning: invalid escape sequence". I've noticed that the Deprecation warning only appears on the first run. It looks like once the code is compiled to `__pycache__`, the deprecation warning does not show. This makes debugging more difficult as I need clean out `__pycache__` directories for the runs to be reproducible.

Example script:

foo.py
```
import bar
```

bar.py
```
s = '\.'
```

First run
```
$ python36 -Wall foo.py 
.../test/bar.py:1: DeprecationWarning: invalid escape sequence \.
  s = '\.'
```

Second run (no DeprecationWarning)
```
$ python36 -Wall foo.py
```

Third run after cleaning
```
$ rm -rf __pycache__
$ python36 -Wall foo.py 
.../test/bar.py:1: DeprecationWarning: invalid escape sequence \.
  s = '\.'
```

I expect the deprecation warning to output on every run.

This warning is emitted by the compiler when it compiles string literals with invalid escape sequences. Once the source is compiled to the bytecode and saved to the .pyc file the compiler no longer involved. You can disable writing compiled bytecode by calling python with option -B.

I see.

I think if the goal is for developers to see and fix these DeprecationWarnings, it would help if the warnings were reproducible without taking steps different from normal Python development. TBH, this is the first time I've ever used the -B CLI argument.

For an example of where developers saw different sets of warnings which lead to confusion, see https://github.com/dateutil/dateutil/pull/358.

This is how Python works. This can't be changed without changing the compiler, the format of .pyc files (for saving warnings) and importing system. And this may slow down normal compilation due to generating warnings for saving them in .pyc files. I don't think this is worth.

Understood. Thanks for the response. I'll have to keep this in mind as I debug these warnings in the future.