diff --git a/build.gradle b/build.gradle
index 3c17d6f20..e13f65690 100644
--- a/build.gradle
+++ b/build.gradle
@@ -200,11 +200,28 @@ rootProject.ext {
   pyver = { exe ->
     try {
       ext.execInBash(
-          exe + " -c 'import sys; print(sys.hexversion)'", "/") as Integer
+          exe + " -c 'import sys; print(sys.hexversion)' 2>/dev/null",
+          "/") as Integer
     } catch (org.gradle.process.internal.ExecException e) {
       return -1;
     }
   }
+
+  // Return the path to a usable python3 executable.
+  getPythonExecutable = {
+    // Find a python version greater than 3.7.3 (this is somewhat arbitrary, we
+    // know we'd like at least 3.6, but 3.7.3 is the latest that ships with
+    // Debian so it seems like that should be available anywhere).
+    def MIN_PY_VER = 0x3070300
+    if (pyver('python') >= MIN_PY_VER) {
+      return 'python'
+    } else if (pyver('/usr/bin/python3') >= MIN_PY_VER) {
+      return '/usr/bin/python3'
+    } else {
+      throw new GradleException("No usable Python version found (build " +
+                                "requires at least python 3.7.3)");
+    }
+  }
 }
 
 task runPresubmits(type: Exec) {
@@ -212,18 +229,7 @@ task runPresubmits(type: Exec) {
   args('config/presubmits.py')
 
   doFirst {
-    // Find a python version greater than 3.7.3 (this is somewhat arbitrary, we
-    // know we'd like at least 3.6, but 3.7.3 is the latest that ships with
-    // Debian so it seems like that should be available anywhere).
-    def MIN_PY_VER = 0x3070300
-    if (pyver('python') >= MIN_PY_VER) {
-      executable 'python'
-    } else if (pyver('/usr/bin/python3') >= MIN_PY_VER) {
-      executable '/usr/bin/python3'
-    } else {
-      throw new GradleException("No usable Python version found (build " +
-                                "requires at least python 3.7.3)");
-    }
+    executable getPythonExecutable()
   }
 }
 
@@ -438,9 +444,10 @@ rootProject.ext {
             ? "${rootDir}/.."
             : rootDir
     def formatDiffScript = "${scriptDir}/google-java-format-git-diff.sh"
+    def pythonExe = getPythonExecutable()
 
     return ext.execInBash(
-        "${formatDiffScript} ${action}", "${workingDir}")
+        "PYTHON=${pythonExe} ${formatDiffScript} ${action}", "${workingDir}")
   }
 }
 
@@ -448,18 +455,23 @@ rootProject.ext {
 // Note that this task checks modified Java files in the entire repository.
 task javaIncrementalFormatCheck {
   doLast {
-    def checkResult = invokeJavaDiffFormatScript("check")
-    if (checkResult == 'true') {
-      throw new IllegalStateException(
-          "Some Java files need to be reformatted. You may use the "
-              + "'javaIncrementalFormatDryRun' task to review\n "
-              + "the changes, or the 'javaIncrementalFormatApply' task "
-              + "to reformat.")
-    } else if (checkResult != 'false') {
-      throw new RuntimeException(
-          "Failed to invoke format check script:\n" + checkResult)
+    // We can only do this in a git tree.
+    if (new File("${rootDir}/.git").exists()) {
+      def checkResult = invokeJavaDiffFormatScript("check")
+      if (checkResult == 'true') {
+        throw new IllegalStateException(
+            "Some Java files need to be reformatted. You may use the "
+                + "'javaIncrementalFormatDryRun' task to review\n "
+                + "the changes, or the 'javaIncrementalFormatApply' task "
+                + "to reformat.")
+      } else if (checkResult != 'false') {
+        throw new RuntimeException(
+            "Failed to invoke format check script:\n" + checkResult)
+      }
+      println("Incremental Java format check ok.")
+    } else {
+      println("Omitting format check: not in a git directory.")
     }
-    println("Incremental Java format check ok.")
   }
 }
 
diff --git a/java-format/google-java-format-diff.py b/java-format/google-java-format-diff.py
index 7cd540447..ebb3558db 100755
--- a/java-format/google-java-format-diff.py
+++ b/java-format/google-java-format-diff.py
@@ -1,4 +1,3 @@
-#!/usr/bin/env python2.7
 #
 #===- google-java-format-diff.py - google-java-format Diff Reformatter -----===#
 #
@@ -24,7 +23,6 @@ For perforce users:
 import argparse
 import difflib
 import re
-import string
 import subprocess
 import io
 import sys
@@ -99,7 +97,7 @@ def main():
     base_command = [binary]
 
   # Reformat files containing changes in place.
-  for filename, lines in lines_by_file.iteritems():
+  for filename, lines in lines_by_file.items():
     if args.i and args.verbose:
       print('Formatting ' + filename)
     command = base_command[:]
@@ -120,11 +118,11 @@ def main():
     if not args.i:
       with open(filename) as f:
         code = f.readlines()
-      formatted_code = io.BytesIO(stdout).readlines()
+      formatted_code = io.StringIO(stdout.decode()).readlines()
       diff = difflib.unified_diff(code, formatted_code,
                                   filename, filename,
                                   '(before formatting)', '(after formatting)')
-      diff_string = string.join(diff, '')
+      diff_string = ''.join(diff)
       if len(diff_string) > 0:
         sys.stdout.write(diff_string)
 
diff --git a/java-format/google-java-format-git-diff.sh b/java-format/google-java-format-git-diff.sh
index 366646626..d5d861db8 100755
--- a/java-format/google-java-format-git-diff.sh
+++ b/java-format/google-java-format-git-diff.sh
@@ -42,6 +42,16 @@ where:
 SCRIPT_DIR="$(realpath $(dirname $0))"
 JAR_NAME="google-java-format-1.8-all-deps.jar"
 
+# Make sure we have a valid python interpreter.
+if [ -z "$PYTHON" ]; then
+  echo "You must specify the name of a python3 interpreter in the PYTHON" \
+       "environment variable."
+  exit 1
+elif ! "$PYTHON" -c ''; then
+  echo "Invalid python interpreter: $PYTHON"
+  exit 1
+fi
+
 # Locate the java binary.
 if [ -n "$JAVA_HOME" ]; then
   JAVA_BIN="$JAVA_HOME/bin/java"
@@ -69,10 +79,14 @@ function runGoogleJavaFormatAgainstDiffs() {
   shift
 
   git diff -U0 "$forkPoint" | \
-      ${SCRIPT_DIR}/google-java-format-diff.py \
-      --java-binary "$JAVA_BIN" \
-      --google-java-format-jar "${SCRIPT_DIR}/${JAR_NAME}" \
-      -p1 "$@" | tee gjf.out
+      "${PYTHON}" "${SCRIPT_DIR}/google-java-format-diff.py" \
+          --java-binary "$JAVA_BIN" \
+          --google-java-format-jar "${SCRIPT_DIR}/${JAR_NAME}" \
+          -p1 "$@" | \
+      tee gjf.out
+
+  # If any of the commands in the last pipe failed, return false.
+  [[ ! "${PIPESTATUS[@]}" =~ [^0\ ] ]]
 }
 
 # Show the file names in a diff preceeded by a message.
@@ -96,7 +110,11 @@ function callGoogleJavaFormatDiff() {
   local callResult
   case "$1" in
     "check")
-      local output=$(runGoogleJavaFormatAgainstDiffs "$forkPoint")
+      # We need to do explicit checks for an error and "exit 1" if there was
+      # one here (though not elsewhere), "set -e" doesn't catch this case,
+      # it's not clear why.
+      local output
+      output=$(runGoogleJavaFormatAgainstDiffs "$forkPoint") || exit 1
       echo "$output" | showFileNames "\033[1mNeeds formatting: "
       callResult=$(echo -n "$output" | wc -l)
       ;;