sidereal.py: Internal maintenance specification

# - - -   p a r s e L o n

def parseLon ( s ):
    """Validate and convert an external longitude.

      [ s is a nonempty string ->
          if s is a valid external longitude ->
            return that longitude in radians
          else -> raise SyntaxError ]
    """

First we'll use Section 8.12, “EW_PAT: Longitude suffix” to validate the suffix letter specifying east or west longitude.

    #-- 1 --
    # [ last  :=  last character of s
    #   rawAngle  :=  s up to the last character ]
    last  =  s[-1]
    rawAngle  =  s[:-1]

    #-- 2 --
    # [ if EW_PAT matches last ->
    #     ewFlag  :=  last, lowercased
    #   else -> raise SyntaxError ]
    m  =  EW_PAT.match ( last )
    if  m is None:
        raise SyntaxError, ( "Longitude '%s' does not end with "
                             "'e' or 'w'." % s )
    else:
        ewFlag  =  last.lower()

Next, check the angle with Section 9.14, “parseAngle(): Convert an external angle string”.

    #-- 3 --
    # [ if rawAngle is a valid angle ->
    #     absAngle  :=  that angle in radians
    #   else -> raise SyntaxError ]
    absAngle  =  parseAngle ( rawAngle )

All that remains is to attach the sign. West Longitude is converted to East Longitude by subtracting from 2π.

    #-- 4 --
    if  ewFlag == 'w':   angle  =  TWO_PI - absAngle
    else:                angle  =  absAngle

    #-- 5 --
    return  angle