Added string format

Author: dbrattli

README.md

@@ -23,8 +23,8 @@ more. Some bindings have already been added:
 Prerequisite for compiling F# to Python using Fable:
 
 ```sh
-> dotnet tool install --global fable-py --version 4.0.0-alpha-004
-> dotnet add package Fable.Core.Experimental --version 4.0.0-alpha-005
+> dotnet tool install --global fable-py --version 4.0.0-alpha-005
+> dotnet add package Fable.Core.Experimental --version 4.0.0-alpha-006
 ```
 
 To use the `Fable.Python` library in your Fable project:

paket.dependencies

@@ -1,3 +1,4 @@
+# https://fsprojects.github.io/Paket/nuget-dependencies.html
 
 source https://api.nuget.org/v3/index.json
 storage: none

src/stdlib/String.fs

@@ -0,0 +1,10 @@
+module Fable.Python.String
+
+open System
+open Fable.Core
+
+// fsharplint:disable MemberNames
+
+type System.String with
+    [<Emit("$0.format($1...)")>]
+    member self.format([<ParamArray>] args: Object[]) = nativeOnly

test/Fable.Python.Test.fsproj

@@ -15,6 +15,7 @@
     <Compile Include="TestAst.fs" />
     <Compile Include="TestBuiltins.fs" />
     <Compile Include="TestJson.fs" />
+    <Compile Include="TestString.fs" />
     <Compile Include="Main.fs" />
   </ItemGroup>
   <Import Project="..\.paket\Paket.Restore.targets" />

test/TestString.fs

@@ -0,0 +1,14 @@
+module Fable.Python.Tests.String
+
+open Util.Testing
+open Fable.Python.String
+
+[<Fact>]
+let ``test string format works`` () =
+    let result = "The sum of 1 + 2 is {0}".format(1+2)
+    result |> equal "The sum of 1 + 2 is 3"
+
+[<Fact>]
+let ``test string format 2 works`` () =
+    let result = "The sum of {0} + 2 is {1}".format(1, 1+2)
+    result |> equal "The sum of 1 + 2 is 3"