__init__ back to nothing and global:

those `copy!(x,y)`s  never worked
(the error was that x and y of different type)

Also: add note for pkg users (i.e. for VoltoMapSim) to set
`__precompile__(false)`, and why.

Author: tfiers

src/Sciplotlib.jl

@@ -1,3 +1,14 @@
+# A note on precompilation and __init__.
+#
+# If Sciplotlib is used in another module, then that module must be marked as
+# `__precompile__(false)`. If not, during precompilation of that top-level module,
+# Sciplotlib's `__init__` will be called [*], which is a function that cannot be called
+# during a precompilation phase; it errors then.
+#
+# [*] Strange but true. Strange cause `__init__` is explicitly made to not be called during
+#     precompilation. It indeed is not called when precompiling Sciplotlib itself. But it
+#     _does_ get called when precompiling a downstream package.
+
 module Sciplotlib
 
 using Reexport

src/init.jl

@@ -1,25 +1,22 @@
-# On `PyNULL()` and `copy!`:
-# https://github.com/JuliaPy/PyCall.jl/issues/699#issuecomment-504616552
-
 """
 Matplotlib's style settings [1]. Note that directly editing `mpl.rcParams` has no effect
 [2]. Editing this object however does work.
 
 - [1] https://matplotlib.org/stable/tutorials/introductory/customizing.html#the-default-matplotlibrc-file
 - [2] https://github.com/JuliaPy/PyPlot.jl#modifying-matplotlibrcparams
 """
-const rcParams = PyNULL()
+rcParams = nothing
 
 """
 A copy of the initial `mpl.rcParams`. Note that we do not use `mpl.rcParamsDefault` or
 `mpl.rcParamsOrig`, as these are different to what's actually used by default (e.g. in a
 Jupyter notebook).
 """
-const rcParams_original = PyNULL()
+rcParams_original = nothing
 
 function __init__()
-    copy!(rcParams_original, mpl.rcParams)
-    copy!(rcParams, PyPlot.PyDict(mpl."rcParams")) # String quotes prevent conversion from
-    #                                              # Python to Julia dict.
+    global rcParams = PyPlot.PyDict(mpl."rcParams")  # String quotes prevent conversion from
+    #                                                # Python to Julia dict.
+    global rcParams_original = copy(mpl.rcParams)
     set_mpl_style!(sciplotlib_style)
 end