Better logging

[atilag]

Welcome contributors!

So far, we've been using a flag called "silent" (you can search for it into the sources), that if set to true, will cause some information to be printed out to the standard output.
We want to change it!
So if you are willing to contribute to this amazing project (I know you are!), I'd suggest you to start with something simple like this issue.
We want to change all the "silent" flag behavior with a more robust and standard logging solution based in the python logging facility. So basically these are the tasks we need to address:

• Incorporate standard python logging module

• Create a logging config file with decent default settings:

  • INFO level
  • Console output
  • Set "disable_exisiting_loggers: false"
  • format: "%(asctime)s : %(name)s : %(levelname)s : %(message)s"

• Loggers should be at module level and use the __name__ attribute:

  • logger = logging.getLogger(__name__)

• Remove "silent" flag everywhere and replace the print() statements with logger.debug() (we don't want to show messages by default)

Suggestions are welcome! :)

Notes

• If you want to take this bug, let us know and we will assign you to it.
• You can open a PR to "master" branch with the word: "WIP" (Work In Progress) in front of the description, so we will know that you haven't finished and we can make parcial reviewings. We will eliminate the WIP prefix once the PR is ready to be merged.
• Make sure all tests pass! :)

Current Behavior

We are using the "silent" variable to print out information via standard output in just some modules. We want to remove them all and use a common logging system.

[hellmersjl3ds]

What's the status of this? I see a commit from @ewinston but it's not assigned to anyone.

I'd be happy to work on it if nobody else is.

[ewinston]

Hi @hellmersjl3ds,
Thanks for offering to have a look at this. In PR#108, referenced in this PR, the python logging facility was setup and covers most of the features requested in this enhancement. One thing I think that remains to be done, however, is that the facility has not been applied everywhere (there are still print statements for instance). If you could help make the logging uniform or suggest any other improvements that would be great. For extensive changes you could open a new issue for discussion or continue using this one.

[1ucian0]

Not every print call in the code should be replaced by log. Some of them need a thought. Following, a list of files in 0.4, with prints in it, and some comments for some of the cases:

• qiskit/tools/visualization.py
• qiskit/tools/qi/qi.py
• qiskit/tools/qi/pauli.py
• qiskit/tools/apps/optimization.py
• qiskit/unroll/_printerbackend.py (probably makes sense to add a file parameter, a la build-in print )
• qiskit/qasm/_qasmlexer.py: the print call in t_error maybe is an Exception (?)
• qiskit/qasm/_node/*.py: Many prints happen in to_string methods. This method should return a string instead of printing (maybe should be called __str__)
• qiskit//qasm/_qasmparser.py: The exception should be re-raised.

Currently, the testsuite do not print any of these issues (I think, it could be that the output is captured in some cases). For sure, tests are needed when performing many of these changes.

This is a very nice opportunity for a first contribution. If anybody wants to work with it and need assistance, let me know! (<- @hellmersjl3ds)

[atilag]

Not every print call in the code should be replaced by log.

The problem of using print()statements, is that it forces the user to dump something to the stdout (usually the monitor), but this is not true for many other uses cases that printing to stdout doesn't make any sense like:

• Graphics User Interfaces (Windows apps, Mac apps, XWindows apps)
• Non-Attended devices
• *nix Daemons and Services
• ... and probably many others I'm not aware of...

We are building an SDK, so using print() is killing many use-cases we don't really want to kill.
So, basically, yes, we should replace al the prints in the code by log :)
Notice that our logger can dump to stdout like print always does, if the user requires to do so... not sure if we are exposing this functionality to the user at the moment, though.

[1ucian0]

Some of the prints I mentioned are messages for an exception, for example.

[hellmersjl3ds]

Hey guys, unfortunately I'm busy with another project for the next 5 weeks or so, so I won't be able to get to this before then.

[atilag]

Ok, so after some PRs... we have included the logging system and we have removed all silent flags.
Still need to remove all presence of print() in the code, but we will create another issue to address this specific issue.
I'm going to close this Issue.