Python TypeError: missing 1 required positional argument — Causes & Fix

What Does This Error Mean?

Python raises TypeError: <function>() missing 1 required positional argument: '<param>' when you call a function or method without supplying all of its required arguments. The interpreter knows exactly which argument is missing because it's declared in the function signature without a default value.

Common Causes (With Code)

1. Calling a function with fewer arguments than it defines

❌ Causes the error

def greet(name, language):
    return f"Hello {name}, you speak {language}"

greet("Alice")
# TypeError: greet() missing 1 required positional argument: 'language'

2. Forgetting self — calling an instance method as a static method

❌ Causes the error

class Dog:
    def bark(self):
        print("Woof!")

Dog.bark()
# TypeError: Dog.bark() missing 1 required positional argument: 'self'

3. Class __init__ missing required arguments on instantiation

❌ Causes the error

class User:
    def __init__(self, username, email):
        self.username = username
        self.email = email

u = User("alice")
# TypeError: User.__init__() missing 1 required positional argument: 'email'

4. Passing a method reference instead of calling it

❌ Causes the error

def process(data, transformer):
    return transformer(data)

def double(x):
    return x * 2

# Accidentally passing two ints instead of a callable
process(5, 10)
# TypeError: 'int' object is not callable — related pattern

How to Fix It

Fix 1 — Pass all required arguments

✅ Correct

def greet(name, language):
    return f"Hello {name}, you speak {language}"

print(greet("Alice", "Python"))
# Hello Alice, you speak Python

Fix 2 — Add a default value to make the argument optional

✅ Correct

def greet(name, language="English"):
    return f"Hello {name}, you speak {language}"

print(greet("Alice"))          # Hello Alice, you speak English
print(greet("Bob", "Python"))  # Hello Bob, you speak Python

Fix 3 — Call instance methods on an instance, not the class

✅ Correct

class Dog:
    def bark(self):
        print("Woof!")

rex = Dog()     # create an instance
rex.bark()      # ✅ Woof!

Fix 4 — Provide all constructor arguments

✅ Correct

class User:
    def __init__(self, username, email):
        self.username = username
        self.email = email

u = User("alice", "alice@example.com")   # ✅
print(u.username)  # alice

Frequently Asked Questions

def connect(host, port=5432, timeout=30):
    ...

connect("localhost")           # ✅ port=5432, timeout=30
connect("localhost", 3306)     # ✅ port=3306, timeout=30