diff --git a/accountant/models/operations.py b/accountant/models/operations.py
index 4c4126b..07326fc 100644
--- a/accountant/models/operations.py
+++ b/accountant/models/operations.py
@@ -24,7 +24,7 @@ from . import db
 
 # pylint: disable=too-many-instance-attributes
 class Operation(db.Model):
-    "Class used to handle operations."
+    """Class used to handle operations."""
     # pylint: disable=invalid-name
     id = db.Column(db.Integer, primary_key=True)
 
@@ -94,6 +94,7 @@ class Operation(db.Model):
         ).label("sold")
     )
 
+    # pylint: disable=too-many-arguments
     def __init__(self, label, value, account_id, operation_date=None,
                  category=None, pointed=False, confirmed=True, canceled=False,
                  scheduled_operation_id=None):
@@ -109,6 +110,7 @@ class Operation(db.Model):
 
     @classmethod
     def query(cls, begin=None, end=None):
+        """Return query for this class."""
         # We have to use a join because the sold is not computed from the
         # begining.
         base_query = db.session.query(
@@ -146,6 +148,7 @@ class Operation(db.Model):
 
     @classmethod
     def get_categories_for_range(cls, account, begin, end):
+        """Get category list for a specific time period."""
         if isinstance(account, int) or isinstance(account, str):
             account_id = account
         else:
@@ -178,6 +181,7 @@ class Operation(db.Model):
 
     @classmethod
     def get_ohlc_per_day_for_range(cls, account, begin=None, end=None):
+        """Get Opening, High, Low, Closing per day for a specific range"""
         if isinstance(account, int) or isinstance(account, str):
             account_id = account
         else: