Check contour levels has correct value for all backends

This commit moves the check for the correctness of the `levels`
keyword argument into `RecipesPipeline.preprocess_attributes!(::KW)`
and calls the new function `check_contour_levels`, which in turn
will throw an `ArgumentError` if the argument is not correct.

To that end:
- Remove the check for non-integer and non-vector values of
  `levels` from the PyPlot backend code, and
- Add low-level tests for the new function and high-level tests
  for contour plots.

src/args.jl

@@ -1502,6 +1502,11 @@ function RecipesPipeline.preprocess_attributes!(plotattributes::AKW)
         plotattributes[:framestyle] = _framestyleAliases[plotattributes[:framestyle]]
     end
 
+    # contours
+    if haskey(plotattributes, :levels)
+        check_contour_levels(plotattributes[:levels])
+    end
+
     # warnings for moved recipes
     st = get(plotattributes, :seriestype, :path)
     if st in (:boxplot, :violin, :density) && !isdefined(Main, :StatsPlots)
@@ -1629,6 +1634,26 @@ convertLegendValue(v::AbstractArray) = map(convertLegendValue, v)
 
 # -----------------------------------------------------------------------------
 
+"""Throw an error if the `levels` keyword argument is not of the correct type
+or `levels` is less than 1"""
+function check_contour_levels(levels)
+    if !(levels isa Union{Integer,AVec})
+        throw(
+            ArgumentError(
+                "the levels keyword argument must be an integer or AbstractVector",
+            ),
+        )
+    elseif levels isa Integer && levels <= 0
+        throw(
+            ArgumentError(
+                "must pass a positive number of contours to the levels keyword argument",
+            ),
+        )
+    end
+end
+
+# -----------------------------------------------------------------------------
+
 # 1-row matrices will give an element
 # multi-row matrices will give a column
 # InputWrapper just gives the contents

src/backends/pyplot.jl

@@ -390,8 +390,6 @@ function py_add_series(plt::Plot{PyPlotBackend}, series::Series)
     elseif isvector(levels)
         extrakw[:levels] = levels
         ()
-    else
-        error("Only numbers and vectors are supported with levels keyword")
     end
 
     # add custom frame shapes to markershape?

test/test_contours.jl

@@ -1,10 +1,46 @@
 using Plots, Test
+import RecipesPipeline
 
 @testset "Contours" begin
+    @testset "check_contour_levels" begin
+        @test Plots.check_contour_levels(2) === nothing
+        @test Plots.check_contour_levels(-1.0:0.2:10.0) === nothing
+        @test Plots.check_contour_levels([-100, -2, -1, 0, 1, 2, 100]) === nothing
+        @test_throws ArgumentError Plots.check_contour_levels(1.0)
+        @test_throws ArgumentError Plots.check_contour_levels((1, 2, 3))
+        @test_throws ArgumentError Plots.check_contour_levels(-3)
+    end
+
+    @testset "RecipesPipeline.preprocess_attributes!" begin
+        function equal_after_pipeline(kw)
+            kw′ = deepcopy(kw)
+            RecipesPipeline.preprocess_attributes!(kw′)
+            kw == kw′
+        end
+
+        @test equal_after_pipeline(KW(:levels => 1))
+        @test equal_after_pipeline(KW(:levels => 1:10))
+        @test equal_after_pipeline(KW(:levels => [1.0, 3.0, 5.0]))
+        @test_throws ArgumentError RecipesPipeline.preprocess_attributes!(
+            KW(:levels => 1.0),
+        )
+        @test_throws ArgumentError RecipesPipeline.preprocess_attributes!(
+            KW(:levels => (1, 2, 3)),
+        )
+        @test_throws ArgumentError RecipesPipeline.preprocess_attributes!(KW(:levels => -3))
+    end
+
+    @testset "contour[f]" begin
         x = (-2π):0.1:(2π)
         y = (-π):0.1:π
         z = cos.(y) .* sin.(x')
 
+        @testset "Incorrect input" begin
+            @test_throws ArgumentError contour(x, y, z, levels = 1.0)
+            @test_throws ArgumentError contour(x, y, z, levels = (1, 2, 3))
+            @test_throws ArgumentError contour(x, y, z, levels = -3)
+        end
+
         @testset "Default number" begin
             @test contour(x, y, z)[1][1].plotattributes[:levels] ==
                   Plots._series_defaults[:levels]
@@ -29,3 +65,4 @@ using Plots, Test
             @test contour(x, y, z, levels = levels)[1][1].plotattributes[:levels] == levels
         end
     end
+end