diff --git a/pkgs/development/libraries/libyaml-cpp/default.nix b/pkgs/development/libraries/libyaml-cpp/default.nix
index 13141923202a..ebfe53b25138 100644
--- a/pkgs/development/libraries/libyaml-cpp/default.nix
+++ b/pkgs/development/libraries/libyaml-cpp/default.nix
@@ -2,13 +2,13 @@
 
 stdenv.mkDerivation rec {
   pname = "libyaml-cpp";
-  version = "0.6.3";
+  version = "0.7.0";
 
   src = fetchFromGitHub {
     owner = "jbeder";
     repo = "yaml-cpp";
     rev = "yaml-cpp-${version}";
-    sha256 = "0ykkxzxcwwiv8l8r697gyqh1nl582krpvi7m7l6b40ijnk4pw30s";
+    sha256 = "sha256-2tFWccifn0c2lU/U1WNg2FHrBohjx8CXMllPJCevaNk=";
   };
 
   # implement https://github.com/jbeder/yaml-cpp/commit/52a1378e48e15d42a0b755af7146394c6eff998c
diff --git a/pkgs/development/tools/build-managers/scons/common.nix b/pkgs/development/tools/build-managers/scons/common.nix
index 6af302851234..da276be7287c 100644
--- a/pkgs/development/tools/build-managers/scons/common.nix
+++ b/pkgs/development/tools/build-managers/scons/common.nix
@@ -1,8 +1,8 @@
 { version, sha256 }:
 
-{ fetchurl, python3Packages, lib }:
+{ fetchurl, python, lib }:
 
-python3Packages.buildPythonApplication rec {
+python.pkgs.buildPythonApplication rec {
   pname = "scons";
   inherit version;
 
@@ -29,6 +29,12 @@ python3Packages.buildPythonApplication rec {
     mv "$out/"*.1 "$out/share/man/man1/"
   '';
 
+  passthru = {
+    # expose the used python version so tools using this (and extensing scos with other python modules)
+    # can use the exact same python version.
+    inherit python;
+  };
+
   meta = with lib; {
     description = "An improved, cross-platform substitute for Make";
     longDescription = ''
diff --git a/pkgs/development/tools/build-managers/scons/default.nix b/pkgs/development/tools/build-managers/scons/default.nix
index 4b07eb0501d8..884d7482faf5 100644
--- a/pkgs/development/tools/build-managers/scons/default.nix
+++ b/pkgs/development/tools/build-managers/scons/default.nix
@@ -1,16 +1,18 @@
-{ callPackage, python2Packages }:
+{ callPackage, python2, python3 }:
 
 let
-  mkScons = args: callPackage (import ./common.nix args) { };
+  mkScons = args: callPackage (import ./common.nix args) {
+    python = python3;
+  };
 in {
   scons_3_0_1 = (mkScons {
     version = "3.0.1";
     sha256 = "0wzid419mlwqw9llrg8gsx4nkzhqy16m4m40r0xnh6cwscw5wir4";
-  }).override { python3Packages = python2Packages; };
+  }).override { python = python3; };
   scons_3_1_2 = (mkScons {
     version = "3.1.2";
     sha256 = "1yzq2gg9zwz9rvfn42v5jzl3g4qf1khhny6zfbi2hib55zvg60bq";
-  }).override { python3Packages = python2Packages; };
+  }).override { python = python2; };
   scons_latest = mkScons {
     version = "4.1.0";
     sha256 = "11axk03142ziax6i3wwy9qpqp7r3i7h5jg9y2xzph9i15rv8vlkj";
diff --git a/pkgs/servers/nosql/mongodb/mongodb.nix b/pkgs/servers/nosql/mongodb/mongodb.nix
index 8a66528d81cd..7c66d348ffb0 100644
--- a/pkgs/servers/nosql/mongodb/mongodb.nix
+++ b/pkgs/servers/nosql/mongodb/mongodb.nix
@@ -1,5 +1,5 @@
 { lib, stdenv, fetchurl, sconsPackages, boost, gperftools, pcre-cpp, snappy, zlib, libyamlcpp
-, sasl, openssl, libpcap, python27, python38, curl, Security, CoreFoundation, cctools }:
+, sasl, openssl, libpcap, curl, Security, CoreFoundation, cctools }:
 
 # Note:
 # The command line tools are written in Go as part of a different package (mongodb-tools)
@@ -12,17 +12,16 @@ with lib;
 
 let
   variants = if versionAtLeast version "4.2"
-    then { python = python38.withPackages (ps: with ps; [ pyyaml cheetah3 psutil setuptools ]);
+    then rec { python = scons.python.withPackages (ps: with ps; [ pyyaml cheetah3 psutil setuptools ]);
             scons = sconsPackages.scons_latest;
             mozjsVersion = "60";
             mozjsReplace = "defined(HAVE___SINCOS)";
           }
-    else { python = python27.withPackages (ps: with ps; [ pyyaml typing cheetah ]);
+    else rec { python = scons.python.withPackages (ps: with ps; [ pyyaml typing cheetah ]);
             scons = sconsPackages.scons_3_1_2;
             mozjsVersion = "45";
             mozjsReplace = "defined(HAVE_SINCOS)";
           };
-  python = python27.withPackages (ps: with ps; [ pyyaml typing cheetah ]);
   system-libraries = [
     "boost"
     "pcre"