diff --git a/esmvalcore/cmor/_fixes/fix.py b/esmvalcore/cmor/_fixes/fix.py
index ba4175813b..4595c3914f 100644
--- a/esmvalcore/cmor/_fixes/fix.py
+++ b/esmvalcore/cmor/_fixes/fix.py
@@ -15,7 +15,7 @@ def __init__(self, vardef):
 
         Parameters
         ----------
-        vardef: basestring
+        vardef: str
             CMOR table entry
 
         """
@@ -31,14 +31,14 @@ def fix_file(self, filepath, output_dir):
 
         Parameters
         ----------
-        filepath: basestring
+        filepath: str
             file to fix
-        output_dir: basestring
+        output_dir: str
             path to the folder to store the fixe files, if required
 
         Returns
         -------
-        basestring
+        str
             Path to the corrected file. It can be different from the original
             filepath if a fix has been applied, but if not it should be the
             original filepath
diff --git a/esmvalcore/cmor/check.py b/esmvalcore/cmor/check.py
index 9fcbb5a62b..48a5113d35 100644
--- a/esmvalcore/cmor/check.py
+++ b/esmvalcore/cmor/check.py
@@ -981,13 +981,13 @@ def cmor_check_metadata(cube,
     ----------
     cube: iris.cube.Cube
         Data cube to check.
-    cmor_table: basestring
+    cmor_table: str
         CMOR definitions to use.
     mip:
         Variable's mip.
-    short_name: basestring
+    short_name: str
         Variable's short name.
-    frequency: basestring
+    frequency: str
         Data frequency.
     check_level: CheckLevels
         Level of strictness of the checks.
@@ -1015,13 +1015,13 @@ def cmor_check_data(cube,
     ----------
     cube: iris.cube.Cube
         Data cube to check.
-    cmor_table: basestring
+    cmor_table: str
         CMOR definitions to use.
     mip:
         Variable's mip.
-    short_name: basestring
+    short_name: str
         Variable's short name
-    frequency: basestring
+    frequency: str
         Data frequency
     check_level: CheckLevels
         Level of strictness of the checks.
@@ -1045,13 +1045,13 @@ def cmor_check(cube, cmor_table, mip, short_name, frequency, check_level):
     ----------
     cube: iris.cube.Cube
         Data cube to check.
-    cmor_table: basestring
+    cmor_table: str
         CMOR definitions to use.
     mip:
         Variable's mip.
-    short_name: basestring
+    short_name: str
         Variable's short name.
-    frequency: basestring
+    frequency: str
         Data frequency.
     check_level: enum.IntEnum
         Level of strictness of the checks.
diff --git a/esmvalcore/cmor/table.py b/esmvalcore/cmor/table.py
index 508ba47a8a..4c5bd2e1ff 100644
--- a/esmvalcore/cmor/table.py
+++ b/esmvalcore/cmor/table.py
@@ -128,7 +128,7 @@ def get_table(self, table):
 
         Parameters
         ----------
-        table: basestring
+        table: str
             Table name
 
         Returns
@@ -144,9 +144,9 @@ def get_variable(self, table_name, short_name, derived=False):
 
         Parameters
         ----------
-        table_name: basestring
+        table_name: str
             Table name
-        short_name: basestring
+        short_name: str
             Variable's short name
         derived: bool, optional
             Variable is derived. Info retrieval for derived variables always
@@ -227,7 +227,7 @@ class CMIP6Info(InfoBase):
 
     Parameters
     ----------
-    cmor_tables_path: basestring
+    cmor_tables_path: str
         Path to the folder containing the Tables folder with the json files
 
     default: object
@@ -367,7 +367,7 @@ def get_table(self, table):
 
         Parameters
         ----------
-        table: basestring
+        table: str
             Table name
 
         Returns
@@ -616,7 +616,7 @@ class CMIP5Info(InfoBase):
 
     Parameters
     ----------
-    cmor_tables_path: basestring
+    cmor_tables_path: str
        Path to the folder containing the Tables folder with the json files
 
     default: object
@@ -757,7 +757,7 @@ def get_table(self, table):
 
         Parameters
         ----------
-        table: basestring
+        table: str
             Table name
 
         Returns
@@ -774,7 +774,7 @@ class CMIP3Info(CMIP5Info):
 
     Parameters
     ----------
-    cmor_tables_path: basestring
+    cmor_tables_path: str
        Path to the folder containing the Tables folder with the json files
 
     default: object
@@ -811,7 +811,7 @@ class CustomInfo(CMIP5Info):
 
     Parameters
     ----------
-    cmor_tables_path: basestring or None
+    cmor_tables_path: str or None
         Full path to the table or name for the table if it is present in
         ESMValTool repository
     """
@@ -848,9 +848,9 @@ def get_variable(self, table, short_name, derived=False):
 
         Parameters
         ----------
-        table: basestring
+        table: str
             Table name
-        short_name: basestring
+        short_name: str
             Variable's short name
         derived: bool, optional
             Variable is derived. Info retrieval for derived variables always