def attr(self, name, value):
"""Add, replace or delete an attribute on selected elements.
If `value` evaulates to `None` the attribute will be deleted from the
element:
>>> html = HTML('<html><head><title>Some Title</title></head>'
... '<body>Some <em class="before">body</em> <em>text</em>.</body>'
... '</html>', encoding='utf-8')
>>> print(html | Transformer('body/em').attr('class', None))
<html><head><title>Some Title</title></head><body>Some <em>body</em>
<em>text</em>.</body></html>
Otherwise the attribute will be set to `value`:
>>> print(html | Transformer('body/em').attr('class', 'emphasis'))
<html><head><title>Some Title</title></head><body>Some <em
class="emphasis">body</em> <em class="emphasis">text</em>.</body></html>
If `value` is a callable it will be called with the attribute name and
the `START` event for the matching element. Its return value will then
be used to set the attribute:
>>> def print_attr(name, event):
... attrs = event[1][1]
... print(attrs)
... return attrs.get(name)
>>> print(html | Transformer('body/em').attr('class', print_attr))
Attrs([(QName('class'), u'before')])
Attrs()
<html><head><title>Some Title</title></head><body>Some <em
class="before">body</em> <em>text</em>.</body></html>
:param name: the name of the attribute
:param value: the value that should be set for the attribute.
:rtype: `Transformer`
"""
return self.apply(AttrTransformation(name, value))
#{ Buffer operations
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