Simpler way to add tagged items in pyasn1

Question

The best way I found to add explicitly tagged items in pyasn1 is to... explicitly tag them. But this looks overly verbose:

cert['tbsCertificate']['extensions'] = rfc2459.Extensions().subtype(explicitTag=tag.Tag(tag.tagClassContext, tag.tagFormatSimple, 3))

Is there any way to generate an empty value which will fit into a place like extensions without specifying the tag?

Answer 1

There is much simpler approach. The convention is that if you assign None to a component of a complex [py]ASN.1 type, that component would be instantiated but will not have a any value.

>>> cert = rfc2459.Certificate()
>>> print cert.prettyPrint()
Certificate:
>>> cert['tbsCertificate'] = None
>>> print cert.prettyPrint()
Certificate:
 tbsCertificate=TBSCertificate:
>>> cert['tbsCertificate']['extensions'] = None
>>> print cert.prettyPrint()
Certificate:
 tbsCertificate=TBSCertificate:
  extensions=Extensions:
>>> cert['tbsCertificate']['extensions'][0] = None
>>> print cert.prettyPrint()
Certificate:
 tbsCertificate=TBSCertificate:
  extensions=Extensions:
   Extension:
>>> cert['tbsCertificate']['extensions'][0]['extnID'] = '1.3.5.4.3.2'
>>> cert['tbsCertificate']['extensions'][0]['extnValue'] = '\x00\x00'
>>> print cert.prettyPrint()
Certificate:
 tbsCertificate=TBSCertificate:
  extensions=Extensions:
   Extension:
    extnID=1.3.5.4.3.2
    extnValue=0x0000
>>> 

That effectively lets you build composite pyasn1 object from either Python built-in or other pyasn1 objects in steps without having to repeat its type specification.