In this article, we will discuss the “sqlalchemy.exc.argumenterror: list argument must consist only of tuples or dictionaries” error that occurs when using SQLAlchemy, a popular Object Relational Mapper (ORM) for Python. We will explore the causes of the error, how to reproduce it, and ultimately, how to resolve it.
Causes of the Error
This error typically arises when you are trying to insert records into a database table using the insert() method with a list of records, but the records are not formatted as tuples or dictionaries. This is a requirement for the insert() method in SQLAlchemy, as it expects a structured input that matches the columns and data types of the target database table.
Reproducing the Error
To reproduce the error, let’s consider the following example. We have a database table called “users” with columns “id”, “name”, and “age”. We will attempt to insert a list of records into the table using the insert() method, but the records will be formatted as lists instead of tuples or dictionaries:
from sqlalchemy import create_engine, Table, Column, Integer, String, MetaData
engine = create_engine('sqlite:///example.db')
metadata = MetaData()
users = Table('users', metadata,
Column('id', Integer, primary_key=True),
Column('name', String),
Column('age', Integer)
)
metadata.create_all(engine)
records = [
["Alice", 30],
["Bob", 25],
["Charlie", 22]
]
with engine.begin() as conn:
conn.execute(users.insert(), records)
This code will trigger the “sqlalchemy.exc.argumenterror: list argument must consist only of tuples or dictionaries” error because the records are formatted as lists instead of tuples or dictionaries.
Resolving the Error
To resolve the error, you need to ensure that the records you are trying to insert are either tuples or dictionaries that match the structure of your database table. Let’s see how to fix the error by formatting the records as tuples:
records = [
("Alice", 30),
("Bob", 25),
("Charlie", 22)
]
with engine.begin() as conn:
conn.execute(users.insert().values([{"name": name, "age": age} for name, age in records]))
In this case, the error is resolved by converting the list of lists into a list of tuples. Alternatively, you can format the records as dictionaries:
records = [
{"name": "Alice", "age": 30},
{"name": "Bob", "age": 25},
{"name": "Charlie", "age": 22}
]
with engine.begin() as conn:
conn.execute(users.insert(), records)
By ensuring that the records are formatted as either tuples or dictionaries
Best Practices to Avoid the Error
To avoid this error in the future, consider following these best practices:
- Always use tuples or dictionaries: When inserting records using SQLAlchemy, always ensure that your data is structured as either tuples or dictionaries, as this is the expected input format for the
insert()method. - Validate data before insertion: Before attempting to insert data into a database table, check the structure and data types of your records to ensure that they match the target table. This can help prevent errors caused by incorrect data formatting or incompatible data types.
- Keep your data model up to date: As you work on your project, your data model may evolve. Ensure that your SQLAlchemy table definitions and data insertion code are updated to reflect any changes to your database schema. This will help prevent errors due to outdated or mismatched data structures.
Conclusion
In conclusion, the “sqlalchemy.exc.argumenterror: list argument must consist only of tuples or dictionaries” error occurs when attempting to insert records into a database table using SQLAlchemy’s insert() method with incorrectly formatted data. By ensuring that your records are structured as either tuples or dictionaries and following the best practices outlined above, you can prevent this error and successfully insert data into your database tables using SQLAlchemy.
