What are Runtime Errors in Python

RuntimeErrors in Python are a type of built-in exception that occurs during the execution of a program. They usually indicate a problem that arises during runtime and is not necessarily syntax-related or caused by external factors.

When an error is detected, and that error doesn't fall into any other specific category of exceptions (or errors), Python throws a runtime error.

Raising a RuntimeError manually

Typically, a Runtime Error will be generated implicitly. But we can raise a custom runtime error manually, using the raise statement. 

Example

In this example, we are purposely raising a RuntimeError using the raise statement to indicate an unexpected condition in the program -

def check_value(x):
   if x < 0:
      raise RuntimeError("Negative value not allowed")

try:
   check_value(-5)
except RuntimeError as e:
   print("RuntimeError caught:", e)

The output is -

RuntimeError caught: Negative value not allowed

When does Python raise RuntimeError?

Python can raise RuntimeError automatically in some situations where an operation fails but does not belong to any specific exception type. But, this is rare, as most errors have dedicated exception classes.

Example: RuntimeError in generators

In this example, a RuntimeError occurs if you try to run a generator that is already executing -

def generator():
   yield 1
   yield 2

gen = generator()
print(next(gen))
print(next(gen))
print(next(gen))  # Raises StopIteration normally

try:
   gen.send(None)  # Will raise RuntimeError: generator already executing if misused
except RuntimeError as e:
   print("RuntimeError caught:", e)

The output depends on how the generator is used, but a RuntimeError can occur during incorrect generator operations -

1
2
Traceback (most recent call last):
  File "/home/cg/root/681af9f2c7256/main.py", line 8, in <module>
    print(next(gen))  # Raises StopIteration normally
          ^^^^^^^^^
StopIteration

Customizing RuntimeErrors

You can raise a RuntimeError explicitly in your own code to show that something unexpected or incorrect has happened, especially when there isn't a more specific error type that fits the situation.

Example

In this example, we flag an error if a function receives an unexpected input -

def process_data(data):
   if not isinstance(data, list):
      raise RuntimeError("Expected a list for processing")
   print("Processing:", data)

try:
   process_data("not a list")
except RuntimeError as e:
   print("RuntimeError caught:", e)

The output is -

RuntimeError caught: Expected a list for processing