I love this purse--not only is it stylish but it's also self-lined. All of which make it an ideal spring/summer purse. I've knit this purse from cotton, raw silk, linen and hemp/cotton blend yarns. 
You will need two balls of worsted weight yarn (approximately 210 yards/192 metres each).
Knitting needles: one set of straight needles 4.50 mm (US 7/UK 7) and two double point needles
Tension: 1" = 5 stitches
To make the body of the purse use the double knit technique.
Cast on 45 stitches
Work in pattern for 13 inches.
Pattern:
Row 1: knit one, *slip stitch as if to purl (bringing yarn forward), knit one, continue from * to last two stitches, slip stitch, knit one
Row 2: knit one, *knit one, slip stitch continue from * to last two stitches, knit two
Repeat rows 1 and 2 for pattern.
This is what the double knit technique looks like on the needles.
and off... 
Cast off two stitches at a time--a knit stitch and a slip stitch treated as one stitch.
To make the strap make I-cord
Cast on 4 to 6 stitches (depending on how thick you want your strap. The more stitches the thicker the strap.)
Work the first row in garter stitch
Once you've knit the first row don't turn your work. Instead, keep the right side of the work facing you and slide the stitches to the other end of the needle. Bring the working yarn forward. Continue working in this manner until you begin to create a "tube". During the initial rows of I-cord, the wrong side may resemble little ladders. Fear not, the laddering will disappear as you continue to work while maintaining tension.
Work the cord to the desired length.
To assemble the purse...
Fold the purse like an envelope. Make the first fold 8 1/2 inches down from the top. The folded piece should be 5 inches deep.
Sew the sides.
Fold the top flap down.
Sew on strap.
(Optional) Attach a button closure.
Weave in ends
