typeerror: __init__() got multiple values for argument 'schema'

ELiv 2025

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06-03-2025

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The dreaded TypeError: __init__() got multiple values for argument 'schema' error often pops up when working with Python classes, particularly those interacting with databases or frameworks like SQLAlchemy or Marshmallow. This error signifies that you've unintentionally provided the schema argument multiple times during the initialization (__init__) of your class. Let's dissect this problem, understand its causes, and learn how to effectively resolve it.

Understanding the Error

The core issue lies in how you're calling the __init__ method of your class. Python's __init__ method is the constructor – it's called when you create a new instance of your class. The error message indicates that you've passed the schema argument twice, either explicitly or implicitly, leading to a conflict for the interpreter. Python doesn't know which value to use, hence the error.

Common Causes and Troubleshooting Steps

Several scenarios can trigger this error. Let's examine them with practical examples and solutions:

1. Duplicate Keyword Arguments

This is the most frequent cause. You might accidentally pass the schema argument twice in your class instantiation:

class MyClass:
    def __init__(self, schema):
        self.schema = schema

# Incorrect instantiation:
my_instance = MyClass(schema="my_schema", schema=another_schema) #Error!

Solution: Carefully review your class instantiation. Ensure you're only passing the schema argument once. Use appropriate variable names to avoid confusion.

# Correct instantiation:
my_instance = MyClass(schema="my_schema") 

2. Conflicting Default Arguments and Explicit Arguments

Suppose your __init__ method has a default value for schema, and you still pass it explicitly:

class MyClass:
    def __init__(self, schema="default_schema"):  #Default schema
        self.schema = schema

# Incorrect instantiation:
my_instance = MyClass(schema="my_schema") #Error! (Schema provided twice)

Solution: If you provide a default value, remove the explicit argument when instantiating, or remove the default value from the __init__ method if you always intend to provide the argument explicitly.

# Correct instantiations:
my_instance = MyClass()  # Uses the default schema
my_instance = MyClass(schema="another_schema")  #Overwrites default

3. Inheritance and Conflicting Arguments

If your class inherits from another class that also takes a schema argument, the conflict might arise during inheritance:

class ParentClass:
    def __init__(self, schema):
        self.schema = schema

class ChildClass(ParentClass):
    def __init__(self, schema, other_arg):
        super().__init__(schema=schema) # Potentially redundant schema
        self.other_arg = other_arg

my_child = ChildClass(schema="my_schema", other_arg="value", schema="another_schema") #Error

Solution: Carefully examine the __init__ methods of both the parent and child classes. Ensure that the schema argument is only passed once in the child class's __init__, typically using super().__init__(schema) only if the parent class requires it. Avoid redundant schema passing in child class __init__.

#Corrected version - no duplicate schema
class ChildClass(ParentClass):
    def __init__(self, schema, other_arg):
        super().__init__(schema=schema) 
        self.other_arg = other_arg

my_child = ChildClass(schema="my_schema", other_arg="value") # Correct

4. Typos or Incorrect Variable Names

Simple typos or inconsistencies in variable names can lead to this error.

Solution: Double-check your spelling and variable names for any discrepancies.

Debugging Tips

• Print Statements: Add print statements within your __init__ method to check the values of the arguments being passed. This can help pinpoint the source of the duplicate argument.

• Interactive Debuggers: Use a debugger (like pdb in Python) to step through your code execution line by line. This will allow you to inspect variable values and identify the exact point where the error occurs.

• Code Review: Have another developer review your code. A fresh pair of eyes can often spot subtle errors that you might have missed.

By understanding the common causes and employing the suggested troubleshooting steps, you can effectively resolve the TypeError: __init__() got multiple values for argument 'schema' and similar errors, ensuring the smooth execution of your Python code. Remember to pay close attention to argument passing during class instantiation and inheritance.