Fix conversion of all-missing column to Pandas (#84)

Author: malmaud

src/tabletraits.jl

@@ -34,6 +34,14 @@ function _construct_pandas_from_iterabletable(source)
         if eltype(v)<:DataValues.DataValue
             T = eltype(eltype(v))
             if T<:AbstractFloat
+
+                # Issue 71
+                # If the column is all 'missing' values, we have to decide what type to give it in Pandas.
+                # We arbitrarily default to Float64.
+                if T == Union{}
+                    T = Float64
+                end
+
                 cols[k] = T[get(i, NaN) for i in v]
             elseif T<:Integer
                 cols[k] = Float64[DataValues.isna(i) ? NaN : Float64(get(i)) for i in v]

test/Project.toml

@@ -1,13 +1,15 @@
 [deps]
 CSV = "336ed68f-0bac-5ca0-87d4-7b16caf5d00b"
-Test = "8dfed614-e22c-5e08-85e1-65c5234f0b40"
+DataFrames = "a93c6f00-e57d-5684-b7b6-d8193f3e46c0"
+DataValues = "e7dc6d0d-1eca-5fa6-8ad6-5aecde8b7ea5"
 IteratorInterfaceExtensions = "82899510-4779-5014-852e-03e436cf321d"
 TableTraits = "3783bdb8-4a98-5b6b-af9a-565f29a5fe9c"
-DataValues = "e7dc6d0d-1eca-5fa6-8ad6-5aecde8b7ea5"
 Tables = "bd369af6-aec1-5ad0-b16a-f7cc5008161c"
+Test = "8dfed614-e22c-5e08-85e1-65c5234f0b40"
 
 [compat]
+DataValues = "0.4.4"
 IteratorInterfaceExtensions = "^0.1.1, ^1"
 TableTraits = "^0.4, ^1"
-DataValues = "0.4.4"
 Tables = "1"
+DataFrames = "1"

test/runtests.jl

@@ -1,5 +1,6 @@
 using Pandas
 using Test
+import DataFrames
 
 df = DataFrame(Dict(:name=>["a", "b"], :age=>[27, 30]))
 age = values(df.age)
@@ -43,3 +44,9 @@ end
 # Rolling
 roll = rolling(Series([1,2,3,4,5]), 3)
 @test isequal(values(mean(roll)), [NaN, NaN, 2.0, 3.0, 4.0])
+
+# Issue #71
+julia_df = DataFrames.DataFrame(x=[1,2], y=[missing, missing])
+py_df = Pandas.DataFrame(julia_df)
+expected_df = Pandas.DataFrame(:x=>[1,2], :y=>[NaN, NaN])
+@test Pandas.equals(py_df, expected_df)