Add support for savestreaming method

Project.toml

@@ -16,9 +16,10 @@ TextParse = "e0df1984-e451-5cb5-8b61-797a481e67e3"
 
 [extras]
 Test = "8dfed614-e22c-5e08-85e1-65c5234f0b40"
+DataFrames = "a93c6f00-e57d-5684-b7b6-d8193f3e46c0"
 
 [targets]
-test = ["Test"]
+test = ["Test", "DataFrames"]
 
 [compat]
 CodecZlib = "≥ 0.5.2"

src/csv_writer.jl

@@ -95,3 +95,24 @@ end
 function fileio_save(s::FileIO.Stream{FileIO.format"TSV"}, data; delim='\t', quotechar='"', escapechar='"', nastring="NA", header=true)
     return _save(s.io, data, delim=delim, quotechar=quotechar, escapechar=escapechar, nastring=nastring, header=header)
 end
+
+#
+# Streaming version writes header (if any) on first call, then appends on subsequent calls.
+#
+const CSV_or_TSV = Union{FileIO.format"CSV", FileIO.format"TSV"}
+
+_delim(T, delim) = (delim === nothing ? (T <: FileIO.format"CSV" ? ',' : '\t') : delim)
+
+function fileio_savestreaming(f::FileIO.File{T}, data; delim=nothing, quotechar='"', escapechar='"', nastring="NA", 
+                              header=true) where T <: CSV_or_TSV
+    io = open(f.filename, "w")
+    _save(io, data; delim=_delim(T, delim), quotechar=quotechar, escapechar=escapechar, nastring=nastring, header=header)
+
+    return FileIO.Stream(T, io, f.filename)
+end
+
+function fileio_savestreaming(s::FileIO.Stream{T}, data; delim=nothing, quotechar='"', escapechar='"', nastring="NA", 
+                              header=false) where T <: CSV_or_TSV
+    return _save(s.io, data; delim=_delim(T, delim), quotechar=quotechar, escapechar=escapechar, nastring=nastring, header=header)
+end
+

test/runtests.jl

@@ -212,8 +212,31 @@ end
         @test showable("text/html", x2) == true
         @test showable("application/vnd.dataresource+json", x2) == true        
     end
+
+end
+
+@testset "savestreaming" begin
+    using DataFrames
 
+    df = DataFrame(A = 1:2:1000, B = repeat(1:10, inner=50), C = 1:500)
+    df1 = df[1:5, :]
+    df2 = df[6:10, :]
+
+    # Test both csv and tsv formats
+    for ext in ("csv", "tsv")
+        fname = "output.$ext"
+        s = savestreaming(fname, df1)
+        savestreaming(s, df2)
+        savestreaming(s, df2)   # add this slice twice
+        close(s)
 
+        new_df = DataFrame(load(fname))
+        @test new_df[1:5,:]   == df1
+        @test new_df[6:10,:]  == df2
+        @test new_df[11:15,:] == df2
+
+        rm(fname)
+    end
 end
 
 end # Outer-most testset