--- a/pygnulib/error.py	Sat Sep 09 23:01:51 2017 +0300
+++ b/pygnulib/error.py	Sat Sep 09 23:41:13 2017 +0300
@@ -16,7 +16,7 @@
                 typeset += [name]
             else:
                 typeset += [module + "." + name]
-        typeset = "{%s}" % ", ".join(set(typeset))
+        typeset = "{0}".format(", ".join(typeset))
         raise TypeError("{0}: {1} expected".format(key, typeset))
 
 
@@ -25,48 +25,42 @@
     """minimum supported autoconf version mismatch"""
     def __init__(self, version):
         fmt = "minimum supported autoconf version is %f"
-        message = fmt % version
-        super().__init__(message)
+        super().__init__(fmt.format(version))
 
 
 
 class M4BaseMismatchError(Exception):
     """<gnulib-comp.m4> is expected to contain gl_M4_BASE([m4base])"""
     def __init__(self, m4_base):
-        fmt = "<gnulib-comp.m4> is expected to contain gl_M4_BASE([%s])"
-        message = fmt % m4_base
-        super().__init__(message)
+        fmt = "<gnulib-comp.m4> is expected to contain gl_M4_BASE([{0}])"
+        super().__init__(fmt.format(m4_base))
 
 
 
 class ConditionalDependenciesUnavailableError(Exception):
     """conditional dependencies are not supported with tests"""
     def __init__(self):
-        message = "conditional dependencies are not supported with tests"
-        super().__init__(message)
+        super().__init__("conditional dependencies are not supported with tests")
 
 
 
 class IncompatibleLicenseError(Exception):
     """incompatible licenses on modules"""
     def __init__(self, modules):
-        fmt = "incompatible licenses on modules: %r"
-        message = fmt % modules
-        super().__init__(message)
+        fmt = "incompatible licenses on modules: {0}"
+        super().__init__(fmt.format(" ".join(modules)))
 
 
 
 class EmptyFileListError(Exception):
     """cannot process empty file list"""
     def __init__(self):
-        message = "cannot process empty file list"
-        super().__init__(message)
+        super().__init__("cannot process empty file list")
 
 
 
 class UnknownLicenseError(Exception):
     """module lacks a license"""
     def __init__(self, module):
-        fmt = "module lacks a license: %r"
-        message = fmt % module
-        super().__init__(message)
+        fmt = "module lacks a license: {0}"
+        super().__init__(fmt.format(module))