Repeatedly calling attr() on same attribute doesn't accumulate values

[cpsievert]

For example, I'd expect the following to generate <button class="bar baz">foo</button>:

from dominate.tags import *

b = button("foo")
with b:
  attr(className = "bar")
  attr(className = "baz")

print(b)

<button class="baz">foo</button>

Accumulating is nice for proving a "base component" with a set of "base classes" that an end user could add to, for example:

def myButton(label):
  b = button(label)
  with b:
    attr(className = "btn")
  return b

b2 = myButton("foo")
with b2:
  attr(className = "btn-primary")

print(b2)

Would ideally generate

<button class="btn btn-primary">foo</button>

[JustinBacher]

So technically your ideal/expected result is actually unexpected behavior. The documentation on the attr function literally says:

Set attributes on the current active tag context

So in essence what you're doing when you call attr(className = "bar") you're quite literally setting className (class) to "btn" then when you call it again with attr(className = "baz") it's literally setting className (class) to "baz".

What you'd really want is the addition of a function along the lines of append_attribute or something, which adds to a current attribute if it exists or falls back to just setting the attribute if it doesn't exist.