[julia-users] error: haskey of NULL PyObject ?

[jda]

I am new to Julia and don't know how to deal with most error messages. 
 Before I could just do 

using PyPlot

and everything worked fine on my windows machine (Julia 0.4.0 repl).  Then 
it broke (possibly due to Pkg.update()? I don't remember at this point.). 
 So I looked up a few things and now it seems that I must do

ENV["PYTHON"]="";
using PyPlot

in order to avoid errors on the "using" line.  But now I have new errors. 
 The command

pcolormesh(rand(20,20))

now gives me the error:

ERROR: ArgumentError: haskey of NULL PyObject
  in pcolormesh at ...\PyPlot.jl:457

All of the above is really strange because on my other mac computer I can 
just do:  using PyPlot; pcolormesh(rand(20,20)); and it's all working fine. 
 I am sure there is something fundamental I am missing about how this works 
on a windows computer, especially since the errors are a new occurrence and 
it was plotting fine recently on the windows.