sidereal.py: Internal maintenance specification

# - - -   p a r s e L a t

def parseLat ( s ):
    """Validate and convert an external latitude.

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

A latitude has two pieces: an angle, and a suffix letter which must be either “n” or “s”, case-insensitive. First we'll peel off the suffix and check it using Section 8.11, “NS_PAT: Latitude suffix”.

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

    #-- 2 --
    # [ if last matches NS_PAT ->
    #     nsFlag  :=  last, lowercased
    #   else -> raise SyntaxError ]
    m  =  NS_PAT.match ( last )
    if  m is None:
        raise SyntaxError, ( "Latitude '%s' does not end with 'n' "
                             "or 's'." % s )
    else:
        nsFlag  =  last.lower()

Validating the angle part is done by 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 )

North latitude is positive, and south latitude is negative.

    #-- 4 --
    if  nsFlag == 's':  angle  =  - absAngle
    else:               angle  =  absAngle

    #-- 5 --
    return angle