diff --git a/.arclint b/.arclint
--- a/.arclint
+++ b/.arclint
@@ -39,6 +39,13 @@
         "lint-tests": {
             "type": "lint-tests",
             "include": "(^src/(rpc/|wallet/)?test/.*\\.(cpp)$)"
+        },
+        "lint-python-format": {
+            "type": "lint-python-format",
+            "include": "(\\.py$)",
+            "exclude": [
+                "(^test/lint/lint-python-format.py)"
+            ]
         }
     }
 }
diff --git a/arcanist/.phutil_module_cache b/arcanist/.phutil_module_cache
--- a/arcanist/.phutil_module_cache
+++ b/arcanist/.phutil_module_cache
@@ -1 +1 @@
-{"__symbol_cache_version__":11,"d60c8224f471e0ecddc2a6f3c6839cd1":{"have":{"class":{"AutoPEP8FormatLinter":75}},"need":{"function":{"pht":297,"id":1317},"class":{"ArcanistExternalLinter":104,"ArcanistLintMessage":1324,"Filesystem":1168,"ArcanistLinter":1431,"ArcanistLintSeverity":1509}},"xmap":{"AutoPEP8FormatLinter":["ArcanistExternalLinter"]}},"213c3145da34ed6dfc0d70d628a2a086":{"have":{"class":{"CheckDocLinter":106}},"need":{"function":{"pht":323,"id":1868},"class":{"ArcanistExternalLinter":129,"ArcanistLintMessage":1875,"Filesystem":737,"ArcanistLinter":1923,"ArcanistLintSeverity":2009}},"xmap":{"CheckDocLinter":["ArcanistExternalLinter"]}},"7bab1f879b8a86dd9977b8c0d075935f":{"have":{"class":{"ClangFormatLinter":79}},"need":{"function":{"pht":302,"execx":787,"id":1664},"class":{"ArcanistExternalLinter":105,"ArcanistLintMessage":1671,"Filesystem":1515,"ArcanistLinter":1778,"ArcanistLintSeverity":1856}},"xmap":{"ClangFormatLinter":["ArcanistExternalLinter"]}},"d5d477360f3d8e940182368e01342fc7":{"have":{"class":{"TestsLinter":106}},"need":{"function":{"pht":321,"id":1943},"class":{"ArcanistExternalLinter":126,"ArcanistLintMessage":1950,"Filesystem":794,"ArcanistLinter":1998,"ArcanistLintSeverity":2084}},"xmap":{"TestsLinter":["ArcanistExternalLinter"]}}}
\ No newline at end of file
+{"__symbol_cache_version__":11,"d60c8224f471e0ecddc2a6f3c6839cd1":{"have":{"class":{"AutoPEP8FormatLinter":75}},"need":{"function":{"pht":297,"id":1317},"class":{"ArcanistExternalLinter":104,"ArcanistLintMessage":1324,"Filesystem":1168,"ArcanistLinter":1431,"ArcanistLintSeverity":1509}},"xmap":{"AutoPEP8FormatLinter":["ArcanistExternalLinter"]}},"213c3145da34ed6dfc0d70d628a2a086":{"have":{"class":{"CheckDocLinter":106}},"need":{"function":{"pht":323,"id":1868},"class":{"ArcanistExternalLinter":129,"ArcanistLintMessage":1875,"Filesystem":737,"ArcanistLinter":1923,"ArcanistLintSeverity":2009}},"xmap":{"CheckDocLinter":["ArcanistExternalLinter"]}},"7bab1f879b8a86dd9977b8c0d075935f":{"have":{"class":{"ClangFormatLinter":79}},"need":{"function":{"pht":302,"execx":787,"id":1664},"class":{"ArcanistExternalLinter":105,"ArcanistLintMessage":1671,"Filesystem":1515,"ArcanistLinter":1778,"ArcanistLintSeverity":1856}},"xmap":{"ClangFormatLinter":["ArcanistExternalLinter"]}},"0e068a1116ed03e86a2388020a821983":{"have":{"class":{"PythonFormatLinter":75}},"need":{"function":{"pht":305,"id":1614},"class":{"ArcanistExternalLinter":102,"ArcanistLintMessage":1621,"Filesystem":733,"ArcanistLinter":1752,"ArcanistLintSeverity":1835}},"xmap":{"PythonFormatLinter":["ArcanistExternalLinter"]}},"ea2beb1668dfbdd87488f18fbb20178f":{"have":{"class":{"TestsLinter":103}},"need":{"function":{"pht":318,"id":2676},"class":{"ArcanistExternalLinter":123,"ArcanistLintMessage":2683,"Filesystem":791,"ArcanistLinter":2731,"ArcanistLintSeverity":2839}},"xmap":{"TestsLinter":["ArcanistExternalLinter"]}}}
\ No newline at end of file
diff --git a/arcanist/__phutil_library_map__.php b/arcanist/__phutil_library_map__.php
--- a/arcanist/__phutil_library_map__.php
+++ b/arcanist/__phutil_library_map__.php
@@ -12,6 +12,7 @@
     'AutoPEP8FormatLinter' => 'linter/AutoPEP8Linter.php',
     'CheckDocLinter' => 'linter/CheckDocLinter.php',
     'ClangFormatLinter' => 'linter/ClangFormatLinter.php',
+    'PythonFormatLinter' => 'linter/PythonFormatLinter.php',
     'TestsLinter' => 'linter/TestsLinter.php',
   ),
   'function' => array(),
@@ -19,6 +20,7 @@
     'AutoPEP8FormatLinter' => 'ArcanistExternalLinter',
     'CheckDocLinter' => 'ArcanistExternalLinter',
     'ClangFormatLinter' => 'ArcanistExternalLinter',
+    'PythonFormatLinter' => 'ArcanistExternalLinter',
     'TestsLinter' => 'ArcanistExternalLinter',
   ),
 ));
diff --git a/arcanist/linter/PythonFormatLinter.php b/arcanist/linter/PythonFormatLinter.php
new file mode 100644
--- /dev/null
+++ b/arcanist/linter/PythonFormatLinter.php
@@ -0,0 +1,97 @@
+<?php
+
+/**
+ * Uses the lint-python-format.py script to update from old % format style to 
+ * new .format().
+ */
+final class PythonFormatLinter extends ArcanistExternalLinter {
+
+  public function getInfoName() {
+    return 'lint-python-format';
+  }
+
+  public function getInfoURI() {
+    return '';
+  }
+
+  public function getInfoDescription() {
+    return pht('Convert python string formatting from % to .format().');
+  }
+
+  public function getLinterName() {
+    return 'lint-python-format';
+  }
+
+  public function getLinterConfigurationName() {
+    return 'lint-python-format';
+  }
+
+  public function getLinterConfigurationOptions() {
+    $options = array();
+
+    return $options + parent::getLinterConfigurationOptions();
+  }
+
+  public function getDefaultBinary() {
+    return Filesystem::resolvePath(
+      'test/lint/lint-python-format.py',
+      $this->getProjectRoot());
+  }
+  
+  public function shouldUseInterpreter() {
+    return true;
+  }
+  
+  public function getDefaultInterpreter() {
+    return "python3";
+  }
+
+  public function getInstallInstructions() {
+    return pht('The test/lint/lint-python-format.py script is part of the bitcoin-abc project');
+  }
+
+  public function shouldExpectCommandErrors() {
+    return false;
+  }
+
+  protected function getMandatoryFlags() {
+    return array();
+  }
+
+  protected function parseLinterOutput($path, $err, $stdout, $stderr) {
+    $pattern = '/\((\d+)\) ([\s\S]+?)=> (.+)/';
+    $found = preg_match_all($pattern, $stdout, $snippets,
+      $flags = PREG_SET_ORDER);
+    
+    /* 
+     * Matched snippets $snippets are organized like this:
+     * [0] The complete mask
+     * [1] The line number
+     * [2] The original snippet
+     * [3] The replacement snippet
+     */
+
+    if (!$found) {
+      return array();
+    }
+
+    $messages = [];
+    foreach($snippets as $snippet) {
+      $messages[] = id(new ArcanistLintMessage())
+        ->setPath($path)
+        ->setLine($snippet[1])
+        ->setChar(1)
+        ->setGranularity(ArcanistLinter::GRANULARITY_FILE)
+        ->setCode('PYFMT')
+        ->setSeverity(ArcanistLintSeverity::SEVERITY_AUTOFIX)
+        ->setName('Old string format notation')
+        ->setDescription("'$path' uses old style string formatting.")
+        ->setOriginalText($snippet[2])
+        ->setReplacementText($snippet[3]);
+    }
+    
+    return $messages;
+  }
+}
+
+?>
\ No newline at end of file