It seems to me that chameleon does not behave properly with boolean attributes:

[ale-rt]

          It seems to me that chameleon does not behave properly with boolean attributes:

[ale@flo chameleon]$ cat test.pt
<input required="" tal:attributes="required string:required">
<input required tal:attributes="required string:required">
<input required foo="bar" tal:attributes="required string:required">

[ale@flo chameleon]$ cat test.py
from chameleon import PageTemplate

with open("test.pt") as f:
    print(PageTemplate(f.read())())

[ale@flo chameleon]$ python test.py 
<input required="required">
<input requiredrequired>
<input requiredrequired foo="bar">

Originally posted by @ale-rt in #119 (comment)

[petschki]

We were bitten by this too in an addon. I've created an issue. see malthe/chameleon#429

[thet]

Did also some tests:

<!--input tal:attributes="required python:False or None"/-->
<input>
<!-- input required tal:attributes="required python:False or None"/-->
<input>
<!-- input required="${python:False or None}" /-->
<input>

<!--input tal:attributes="required python:True or None"/-->
<input required="required">
<!-- input required tal:attributes="required python:True or None"/-->
<input requiredrequired="">
<!-- input required="${python:True or None}" /-->
<input required="required">

I prefer the form <input required="${python:True or None}" />.

Still the issue in Chameleon needs to be fixed.