Dry run mode

[cphyc]

This adds a dry-run mode and fixes #48.

[ikamensh]

why -n? I am using the first letter as a short-hand for each flag; -d would be more consistent (and intuitive imo)

[ikamensh]

Perhaps it's useful to elaborate that it MUST be used with --fail-on-change, otherwise flynt just does nothing and returns nothing or potentially fails.

[ikamensh]

Thanks for your contribution!
few remarks:

1. Issue feature request: dryrun mode? #48 was proposing to e.g. print the diff to the stdout, which this version of the feature is not doing yet.
   would you like to expand this contribution to print the diff?

2. let's print a message at the beginning stating the changed behavior. Something like "Flynt is running in dry mode. Files will not be changed." Reasoning: prevent any confusion.

[cphyc]

Thanks for the feedback! Does this looks reasonable to you?

• I added a small message when running in dry-run mode to inform no files will be touched
• a diff-style message is printed instead of writing to files
• the user is advised to use --fail-on-change when using --dry-run for linting purposes.

For example on the CI of https://github.com/yt-project/yt, this produces

$ flynt --fail-on-change --dry-run yt/
Running flynt v.0.49
Running flynt in dry-run mode. No files will be changed.
--- /home/travis/build/yt-project/yt/yt/exthook.py
+++ 
@@ -86,7 +86,7 @@
             if "." not in modname:
                 setattr(sys.modules[self.wrapper_module], modname, module)
             return module
-        raise ImportError("No module named %s" % fullname)
+        raise ImportError(f"No module named {fullname}")
 
     def is_important_traceback(self, important_module, tb):
         """Walks a traceback's frames and checks if any of the frames
[...]
flynt run has finished. Stats:
Execution time:                            11.892s
Files modified:                            205
Character count reduction:                 4773 (0.08%)
Per expression type:
Old style (`%`) expressions attempted:     1038/1298 (80.0%)
`.format(...)` calls attempted:            125/126 (99.2%)
No concatenations attempted.
F-string expressions created:              1081
Out of all attempted transforms, 248 resulted in errors.
To find out specific error messages, use --verbose flag.
_-_._-_._-_._-_._-_._-_._-_._-_._-_._-_._-_._-_._-_._-_._-_._-_._-_._-_._-_._-_._-_._-_._-_._-_._-_.
Please run your tests before committing. Did flynt get a perfect conversion? give it a star at: 
~ https://github.com/ikamensh/flynt ~
Thank you for using flynt. Upgrade more projects and recommend it to your colleagues!
The command "flynt --fail-on-change --dry-run yt/" exited with 205.

[ikamensh]

Thumbs up on picking up the diff part as well. I didn't know there was a stdlib module for handling this!

I have tried this and it works nicely. Ready to merge it, with the small comment about readme - now that we print diff the --dry-run became useful on it's own.

[ikamensh]

When suggesting this change, I didn't consider we will end up implementing printing diff. Now it's more relevant to state that proposed diff will be printed to stdout, and --dry-run because useful on it's own without --fail-on-change.

[cphyc]

This should be fixed now.

[ikamensh]

Thanks! merged.

[cphyc]

Do you think it would be possible to issue a release (v.50.0)?

[ikamensh]

done: https://pypi.org/project/flynt/0.50/