<h4 dir="auto">Description</h4>
<p dir="auto">The documentation of default values in many classes is either not included, inconsistent in how it is written, or out-of-date. I would like to gather a few people to work on the default value documentation for every class as there are a ton of classes where these issues exist. I have been told that the default values should be documented as "default=&lt;'value'&gt;" and so I am creating this issue under that assumption.</p>
<h4 dir="auto">Solution</h4>
<p dir="auto">Here are a few things that I have seen for the parameters which should be changed:</p>
<ul dir="auto">
<li>no mention of whether there is a default should be checked against code as a few parameters are missing this entirely</li>
<li>"optional" should be changed to "default=&lt;'value'&gt;"</li>
<li>make sure how the default values are documented is consistent within the class, i.e. change everything to the format "default=&lt;'value'&gt;"</li>
<li>Modify a single file per PR</li>
</ul>
<p dir="auto">If a few people work on a few classes each, then this should be done in no time! These should all be fairly simple fixes.</p>
<h4 dir="auto">Examples</h4>
<p dir="auto"><a href="https://scikit-learn.org/stable/modules/generated/sklearn.cluster.AgglomerativeClustering.html" rel="nofollow">https://scikit-learn.org/stable/modules/generated/sklearn.cluster.AgglomerativeClustering.html</a><br>
The link above is an example where default values are not indicated but the parameters say "optional", and where those with default values indicated are all inconsistently documented.</p>
<h4 dir="auto">Describe the workflow you want to enable</h4>
<p dir="auto">It would be convenient if after each step of the pipeline you could call a custom function (callback) if needed.<br>
For example, in simple TF-IDF + LogReg pipeline, after the step of fitting TF-IDF, I would like to look at the dimension of the resulting dictionary (output it to stdout).</p>
<h4 dir="auto">Describe your proposed solution</h4>
<p dir="auto">Add additional parameter <code class="notranslate">callbacks</code> to Pipeline, which takes List of (name, transform) tuples, similar to the <code class="notranslate">steps</code> parameter (default: None).</p>