I just discovered a logical bug in my code which was causing all sorts of problems. I was inadvertently doing a bitwise AND instead of a logical AND.

I changed the code from:

r = mlab.csv2rec(datafile, delimiter=',', names=COL_HEADERS)
mask = ((r["dt"] >= startdate) & (r["dt"] <= enddate))
selected = r[mask]

TO:

r = mlab.csv2rec(datafile, delimiter=',', names=COL_HEADERS)
mask = ((r["dt"] >= startdate) and (r["dt"] <= enddate))
selected = r[mask]

To my surprise, I got the rather cryptic error message:

  
ValueError: The truth value of an array with more than one element is
  ambiguous. Use a.any() or a.all()

Why was a similar error not emitted when I use a bitwise operation - and how do I fix this?