diff --git a/patacrep/build.py b/patacrep/build.py
index 7ec0d739..6c61583b 100644
--- a/patacrep/build.py
+++ b/patacrep/build.py
@@ -128,9 +128,9 @@ class Songbook:
     def iter_errors(self):
         """Iterate over errors of book and book content."""
         yield from self._errors
-        content = self._config.get('content', list())
-        yield from content.iter_errors()
-        for item in content:
+        contentlist = self._config.get('content', list())
+        yield from contentlist.iter_errors()
+        for item in contentlist:
             if not hasattr(item, "iter_errors"):
                 continue
             yield from item.iter_errors()
diff --git a/patacrep/content/__init__.py b/patacrep/content/__init__.py
index ef389bfd..1aaf5043 100755
--- a/patacrep/content/__init__.py
+++ b/patacrep/content/__init__.py
@@ -147,28 +147,38 @@ class ContentList:
         yield from self._content
 
     def extend(self, iterator):
+        """Extend content list with an iterator.
+
+        If the argument is of the same type, the list of errors is
+        also extended.
+        """
         self._content.extend(iterator)
-        if isinstance(iterator, self.__class__):
-            self._errors.extend(iterator._errors)
+        if isinstance(iterator, ContentList):
+            self._errors.extend(iterator.iter_errors())
 
     def append(self, item):
+        """Append an item to the content list."""
         return self._content.append(item)
 
     def __len__(self):
         return len(self._content)
 
     def append_error(self, error):
+        """Log and append an error to the error list."""
         LOGGER.warning(error)
         self._errors.append(error)
 
     def extend_error(self, errors):
+        """Extend the error list with the argument, which is logged."""
         for error in errors:
             self.append_error(error)
 
     def iter_errors(self):
+        """Iterate over errors."""
         yield from self._errors
 
 class EmptyContentList(ContentList):
+    """Empty content list: contain only errors."""
     def __init__(self, *, errors):
         super().__init__()
         for error in errors:
diff --git a/patacrep/content/include.py b/patacrep/content/include.py
index a972d5bf..92f060ab 100644
--- a/patacrep/content/include.py
+++ b/patacrep/content/include.py
@@ -6,7 +6,6 @@ songs in JSON format.
 
 import json
 import os
-import sys
 import logging
 
 from patacrep.content import process_content, ContentError, ContentList
diff --git a/patacrep/content/song.py b/patacrep/content/song.py
index a2cce2c0..dbdbe6c8 100755
--- a/patacrep/content/song.py
+++ b/patacrep/content/song.py
@@ -91,8 +91,8 @@ def parse(keyword, argument, contentlist, config):
                     extension = filename.split(".")[-1]
                     if extension not in plugins:
                         songlist.append_error(ContentError(message=(
-                                'I do not know how to parse "{}": name does '
-                                'not end with one of {}. Ignored.'
+                            'I do not know how to parse "{}": name does '
+                            'not end with one of {}. Ignored.'
                             ).format(
                                 os.path.join(songdir.datadir, filename),
                                 ", ".join(["'.{}'".format(key) for key in plugins.keys()]),
diff --git a/patacrep/content/songsection.py b/patacrep/content/songsection.py
index 2a5ba2d5..c5fea4ab 100755
--- a/patacrep/content/songsection.py
+++ b/patacrep/content/songsection.py
@@ -41,7 +41,7 @@ def parse(keyword, argument, contentlist, config):
 
 
 CONTENT_PLUGINS = dict([
-    (word, parse)
-    for word
+    (keyword, parse)
+    for keyword
     in KEYWORDS
     ])
diff --git a/patacrep/content/sorted.py b/patacrep/content/sorted.py
index cdb0dcd2..9f9fded9 100755
--- a/patacrep/content/sorted.py
+++ b/patacrep/content/sorted.py
@@ -9,7 +9,7 @@ import logging
 import unidecode
 
 from patacrep import files
-from patacrep.content import ContentError, ContentList, EmptyContentList
+from patacrep.content import ContentError, EmptyContentList
 from patacrep.content import process_content
 from patacrep.content.song import OnlySongsError
 
diff --git a/patacrep/content/tex.py b/patacrep/content/tex.py
index efacb838..f32880cf 100755
--- a/patacrep/content/tex.py
+++ b/patacrep/content/tex.py
@@ -51,11 +51,12 @@ def parse(keyword, argument, contentlist, config):
                     ))
                 break
         if not checked_file:
-            filelist.append_error(ContentError(
-                keyword="tex",
-                message=errors.notfound(filename, basefolders),
+            filelist.append_error(
+                ContentError(
+                    keyword="tex",
+                    message=errors.notfound(filename, basefolders),
+                    )
                 )
-            )
             continue
         filelist.append(LaTeX(checked_file))
 
diff --git a/patacrep/latex/syntax.py b/patacrep/latex/syntax.py
index 890ca79c..edf9ceb1 100644
--- a/patacrep/latex/syntax.py
+++ b/patacrep/latex/syntax.py
@@ -3,11 +3,10 @@
 import logging
 import ply.yacc as yacc
 
-from patacrep.songs.syntax import Parser
-from patacrep.latex.lexer import tokens, SimpleLexer, SongLexer
 from patacrep.latex import ast
-from patacrep.errors import ParsingError
 from patacrep.latex.detex import detex
+from patacrep.latex.lexer import tokens, SimpleLexer, SongLexer
+from patacrep.songs.syntax import Parser
 
 LOGGER = logging.getLogger()
 
@@ -126,8 +125,7 @@ class LatexParser(Parser):
         else:
             symbols[0] = []
 
-    @staticmethod
-    def p_dictionary(symbols):
+    def p_dictionary(self, symbols):
         """dictionary : identifier EQUAL braces dictionary_next
                       | identifier EQUAL error dictionary_next
                       | empty
diff --git a/patacrep/songs/chordpro/syntax.py b/patacrep/songs/chordpro/syntax.py
index 7fcf5d4b..8c17b3da 100644
--- a/patacrep/songs/chordpro/syntax.py
+++ b/patacrep/songs/chordpro/syntax.py
@@ -1,6 +1,5 @@
 """ChordPro parser"""
 
-import functools
 import logging
 import re