The only position field in GFA1 is the pos field in the C lines. This represents the starting position of the contained segment in the container segment and is 0-based.

Some fields in GFA2 E lines (beg1, beg2, end1, end2) and F lines (s_beg, s_end, f_beg, f_end) are positions. According to the specification, they are 0-based and represent virtual ticks before and after each string in the sequence. Thus ranges are represented similarly to the Python range conventions: e.g. a 1-character prefix of a sequence will have begin 0 and end 1.

Last positions in GFA2

The GFA2 positions must contain an additional string ($) appended to the integer, if (and only if) they are the last position in the segment sequence. These particular positions are represented in Gfapy as instances of the class LastPos.

To create a lastpos instance, the constructor can be used with an integer, or the string representation (which must end with the dollar sign, otherwise an integer is returned):

>>> str(gfapy.LastPos(12))
>>> gfapy.LastPos("12")
>>> str(gfapy.LastPos("12"))
>>> gfapy.LastPos("12$")
>>> str(gfapy.LastPos("12$"))

Subtracting an integer from a lastpos returns a lastpos if 0 subtracted, an integer otherwise. This allows to do some arithmetic on positions without making them invalid.

>>> gfapy.LastPos(12) - 0
>>> gfapy.LastPos(12) - 1

The functions islastpos() and isfirstpos() allow to determine if a position value is 0 (first), or the last position, using the same syntax for lastpos and integer instances.

>>> gfapy.isfirstpos(0)
>>> gfapy.islastpos(0)
>>> gfapy.isfirstpos(12)
>>> gfapy.islastpos(12)
>>> gfapy.islastpos(gfapy.LastPos("12"))
>>> gfapy.islastpos(gfapy.LastPos("12$"))