[lxml] Why the character apostrophe is not escaped?

I was wondering why the apostrophe is not automatically escaped as other characters? For example: < is transformed to *<* and

is transformed to *>* but ' is not transformed to *'*

Python : sys.version_info(major=2, minor=7, micro=6, releaselevel='final', serial=0) lxml.etree : (3, 3, 3, 0) libxml used : (2, 9, 1) libxml compiled : (2, 9, 1) libxslt used : (1, 1, 28) libxslt compiled : (1, 1, 28)