diff --git a/Library/Formula/python.rb b/Library/Formula/python.rb
index ec0892e3fe8b..28321e501641 100644
--- a/Library/Formula/python.rb
+++ b/Library/Formula/python.rb
@@ -110,18 +110,20 @@ def install
 
     args << "--without-gcc" if ENV.compiler == :clang
 
+    cflags   = []
+    ldflags  = []
+    cppflags = []
+
     unless MacOS::CLT.installed?
       # Help Python's build system (setuptools/pip) to build things on Xcode-only systems
       # The setup.py looks at "-isysroot" to get the sysroot (and not at --sysroot)
-      cflags = "CFLAGS=-isysroot #{MacOS.sdk_path}"
-      ldflags = "LDFLAGS=-isysroot #{MacOS.sdk_path}"
-      args << "CPPFLAGS=-I#{MacOS.sdk_path}/usr/include" # find zlib
+      cflags   << "-isysroot #{MacOS.sdk_path}"
+      ldflags  << "-isysroot #{MacOS.sdk_path}"
+      cppflags << "-I#{MacOS.sdk_path}/usr/include" # find zlib
       # For the Xlib.h, Python needs this header dir with the system Tk
       if build.without? "tcl-tk"
-        cflags += " -I#{MacOS.sdk_path}/System/Library/Frameworks/Tk.framework/Versions/8.5/Headers"
+        cflags << "-I#{MacOS.sdk_path}/System/Library/Frameworks/Tk.framework/Versions/8.5/Headers"
       end
-      args << cflags
-      args << ldflags
     end
 
     # Avoid linking to libgcc https://code.activestate.com/lists/python-dev/112195/
@@ -157,10 +159,14 @@ def install
 
     if build.with? "tcl-tk"
       tcl_tk = Formula["homebrew/dupes/tcl-tk"].opt_prefix
-      ENV.append "CPPFLAGS", "-I#{tcl_tk}/include"
-      ENV.append "LDFLAGS", "-L#{tcl_tk}/lib"
+      cppflags << "-I#{tcl_tk}/include"
+      ldflags  << "-L#{tcl_tk}/lib"
     end
 
+    args << "CFLAGS=#{cflags.join(' ')}"     unless cflags.empty?
+    args << "LDFLAGS=#{ldflags.join(' ')}"   unless ldflags.empty?
+    args << "CPPFLAGS=#{cppflags.join(' ')}" unless cppflags.empty?
+
     system "./configure", *args
 
     # HAVE_POLL is "broken" on OS X. See:
diff --git a/Library/Formula/python3.rb b/Library/Formula/python3.rb
index 580e1518ddbd..7105647d9216 100644
--- a/Library/Formula/python3.rb
+++ b/Library/Formula/python3.rb
@@ -109,17 +109,20 @@ def install
 
     args << "--without-gcc" if ENV.compiler == :clang
 
+    cflags   = []
+    ldflags  = []
+    cppflags = []
+
     unless MacOS::CLT.installed?
       # Help Python's build system (setuptools/pip) to build things on Xcode-only systems
       # The setup.py looks at "-isysroot" to get the sysroot (and not at --sysroot)
-      cflags = "CFLAGS=-isysroot #{MacOS.sdk_path}"
-      ldflags = "LDFLAGS=-isysroot #{MacOS.sdk_path}"
-      args << "CPPFLAGS=-I#{MacOS.sdk_path}/usr/include" # find zlib
+      cflags   << "-isysroot #{MacOS.sdk_path}"
+      ldflags  << "-isysroot #{MacOS.sdk_path}"
+      cppflags << "-I#{MacOS.sdk_path}/usr/include" # find zlib
+      # For the Xlib.h, Python needs this header dir with the system Tk
       if build.without? "tcl-tk"
-        cflags += " -I#{MacOS.sdk_path}/System/Library/Frameworks/Tk.framework/Versions/8.5/Headers"
+        cflags << "-I#{MacOS.sdk_path}/System/Library/Frameworks/Tk.framework/Versions/8.5/Headers"
       end
-      args << cflags
-      args << ldflags
     end
     # Avoid linking to libgcc http://code.activestate.com/lists/python-dev/112195/
     args << "MACOSX_DEPLOYMENT_TARGET=#{MacOS.version}"
@@ -150,10 +153,14 @@ def install
 
     if build.with? "tcl-tk"
       tcl_tk = Formula["tcl-tk"].opt_prefix
-      ENV.append "CPPFLAGS", "-I#{tcl_tk}/include"
-      ENV.append "LDFLAGS", "-L#{tcl_tk}/lib"
+      cppflags << "-I#{tcl_tk}/include"
+      ldflags  << "-L#{tcl_tk}/lib"
     end
 
+    args << "CFLAGS=#{cflags.join(' ')}"     unless cflags.empty?
+    args << "LDFLAGS=#{ldflags.join(' ')}"   unless ldflags.empty?
+    args << "CPPFLAGS=#{cppflags.join(' ')}" unless cppflags.empty?
+
     system "./configure", *args
 
     system "make"