How can I set the dash length in a matplotlib contour plot

Almost.

It's:

for c in CS.collections:
    c.set_dashes([(0, (2.0, 2.0))])

If you had put a print c.get_dashes() there, you would have found out (it's what I did).

Perhaps the definition of the line style has changed a bit, and you were working from an older example.

The collections documentation has this to say:

• set_dashes(ls)

  alias for set_linestyle

• set_linestyle(ls)

  Set the linestyle(s) for the collection.

  ACCEPTS: [‘solid’ | ‘dashed’, ‘dashdot’, ‘dotted’ | (offset, on-off-dash-seq) ]

So in [(0, (2.0, 2.0))], 0 is the offset, and then the tuple is the on-off repeating pattern.