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This is a Chainer's Trainer extension that reports the same contents as the commonly-used PrintReport extension to Slack.

from chainer_slack_report import SlackReport
r = SlackReport(os.environ.get("SLACK_ACCESS_TOKEN", None),
                os.environ.get("SLACK_CHANNEL_ID", None),
                ['epoch', 'main/loss', 'validation/main/loss',
                 'main/accuracy', 'validation/main/accuracy', 'elapsed_time'],
trainer.extend(r, trigger=(1, 'epoch'))



Collision-free Slack messages

The key feature (and difference between similar tools) of SlackReport is that it can handle multiple training streams without collapsing Slack screen, as shown in the above screencast.

In case you run many training processes in parallel and each of them uses SlackReport, the reported contents of each process are always summarized in a single message posted by a bot, without being mixed up with other processes'.

That makes you easily track how each process is going on.

Slack message length limit aware

Basically Slack limits message length to be less than 4000 bytes, but training reports can easily exceed it if you send everything. chainer-slack-report automatically deals with this by a strategy you specified to len_normalizer arguments.

  • "thin_out": (default) Skip a few lines uniformly, by automatically determine an optimal interval
  • "FIFO": Throw the oldest lines away
  • "LIFO": Stop new contents to be added

The first line of the content, which is table header, is always kept on top regardless of the storategy.

Send a mention after the training finishes

You can pass Slack user IDs that begin with '@' to the finish_mentions keyword argument of the SlackReport initializer as either a str or a list, in order to let SlackReport send a mentioning message after finishing the training process, so that you can immediately notice the training has finished.
If nothing is specified to finish_mentions, it just shows the final log.

Custom report format

You can customize what information to report to Slack by specifying a format string to the template argument.

                template="{status} {content}")

Named placeholders ("{...}") are available, which are listed below, and the template is processed by Python's standard string formatter (str.format_map). In case an unknown placeholder is specified, it'll be kept as it is with a warning.

  • {status}: Status string (eg. [Completed], [Training])
  • {hostname}: Hostname of where the training is running on
  • {pwd}: Current directory (full path)
  • {pwdshort}: Current directory whose home part is replaced by ~
  • {cmd}: argv[0]
  • {args}: argv[1:]
  • {elapsed}: Elapsed time from the SlackReport instance is created (in sec)
  • {content}: Main content of the reports
  • {finish_mentions}: Slack mention (appears after the training finished)


% pip install chainer-slack-report


% pip install git+

Slack App preparation

In order to get it work, you will first need to prepare for a bot user on your Slack workspace as explained in the following instruction.

Access to You may be asked for signing-in to a workspace. Then you will see a window with the "Create New App" button which you should click. Create new app

Fill the App Name field. Anything is fine. The workspace should be the one that you'd like to send reports to. Then click the "Create App" button. Set app name and workspace

Next, you have to tell Slack that you want the app to behave as a Bot. Scroll down the window and click the "Bots" button. Go to bot creation screen

Just click the "Add a Bot User" button. Here, Slack knows that your app is a bot. Create a bot

You can name arbitrary display name and user name to the bot, except that the user name has to be valid as an identifier. Create a bot

Next step is to activate the bot in your workspace in order to get a credential to access to Slack API. Go to "Install App" screen. Install app

Install app

Then you'll be redirected to a screen with a bot Access Token (the bottom one). This is what you need to pass to SlackReport. Token

By the way, your bot doesn't have an icon yet, so let's set it from the "Basic Information" screen. Basic Information Set icon

Here, your bot is ready!

You also have to identify Slack channel ID to send report to.

On the Slack screen, you can find the channel ID by looking at full URL of the target channel.

Channel URL

The copied URL will be like, whose CXXXXXXXX part is the channel ID.

How it works

SlackReport inherits extension, and hooks its print function, and just sends the content to Slack rather than stdout.

Therefore, except for out argument, you can pass anything that PrintReport can accept to SlackReport too.

At the very beginning of the SlackReport object lifetime, it "creates" a new post, and from next time it "updates" the previous post.


MIT License


Sends multiple Chainer training histories to Slack without corrupting chronological order






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