As you do more and more programming, you will naturally encounter a lot of errors (or bugs). Causing, understanding, and fixing errors is an important part of programming. Python will do its best to run anything that you tell it to run, but if it can't understand what you're asking, then it won't run the program. All the same, Python will try to tell you a little bit of information about what went wrong, in order to help you try to fix it.
Here are two Python errors.
Example: A Syntax Error
In this first example, we forget to use the parenthesis that are required by
Here is a second example of a bug in Python.
Example: A Run-Time Error
In the second example, we forget to define the
A syntax error happens when Python can't understand what you are saying. A run-time error happens when Python understands what you are saying, but runs into trouble when following your instructions.
In English, a syntax error would be like the sentence
Please cat dog monkey.
The grammar of this sentence does not make sense. From the perspective of English grammar, it is missing a verb (action). You cannot understand what you are being asked to do. Syntax means the same thing as grammar.
In English, a run-time error would be like the sentence
Please eat the piano.
The sentence makes sense from a grammatical perspective — there is a verb and noun in the right place — so you know what you are being asked to do. But, you will encounter problems once you start trying to eat the piano (except maybe if you are a termite). This is called a run-time error because it occurs after the program starts running.
We also will talk about logic errors at the end, which means that your program runs without crashing, but still produces the wrong result. An example would be
Please close the back door so that the bugs don't come in.
This would be a logic error if the front door is also open: even though the command makes sense and you can close the back door, this doesn't actually accomplish the goal of keeping the bugs out since they'll still come in the front door.
Common Syntax Errors in Python
Here are a few additional examples of syntax errors that can occur in Python. One very general thing that can occur is that Python will encounter a special symbol in a place that it doesn't expect.
Example: Syntax Error
Here is another syntax error that is more subtle.
Example: Syntax Error
The problem is that
If you are using quotes around text and you forget the second one, or you are using parentheses and forget the second one, you will get syntax errors:
Example: Syntax ErrorForgetting the second quote
In this error, EOL is short for End Of Line: Python expected another
Example: Syntax ErrorForgetting the second parenthesis
Similarly, EOF is short for End Of File: Python kept looking for a
Sometimes two very similar syntax errors can give two very different error messages. But, every error message is indeed trying to tell you something helpful.
Here are a few common run-time errors. Python is able to understand what the program says, but runs into problems when actually performing the instructions.
You will find more ways of producing errors as you learn more about Python.
What is the technical difference between syntax and run-time errors? Here is an example comparing a run-time error to a syntax error. Look at the output of each program.
Example: Run-Time Error
Example: Syntax ErrorThe program with the run-time error created some output, but the one with the syntax error did not. This is because Python runs in two steps:
So, a program with a syntax error will execute no steps at all, but a program with a run-time error will execute the steps that happened before the error occured.
Your program might run without crashing (no syntax or run-time errors), but still do the wrong thing. For example, perhaps you want a program to calculate the average of two numbers: the average of x and y is defined as
Why doesn't this program work?
ExampleThis does not calculate the average correctly.
The average should be
but the program prints
You can have logic errors because you designed a program incorrectly, or because you didn't write code that follows the design correctly (like the
Now that the lesson is complete, we have three exercises on debugging (fixing errors in programs). You can try to spot the errors before running the programs, or you can run them first and use Python's response to determine what needs fixing.
What is RuntimeError in Python?
A runtime error is a type of error that occurs during program execution. The Python interpreter executes a script if it is syntactically correct. However, if it encounters an issue at runtime, which is not detected when the script is parsed, script execution may halt unexpectedly.
How do I get rid of runtime error in Python?
How can I fix Python runtime error?.
Use an undefined variable or function. This can also occur if you use capital letters inconsistently in a variable name: callMe = “Brad” ... .
Dividing by zero. Guess what? ... .
Use operators on the wrong type of data. print(“you are trying to add text and numbers” + 20).
When to use RuntimeError?
A RuntimeError is raised when attempting to perform an invalid operation of some kind, which doesn't meet the criteria of a more specific error classification.
What is an example of a runtime error?
A program crash is the most noticeable type of runtime error since the program unexpectedly quits while running. Crashes can be caused by memory leaks or other programming errors. Common examples include dividing by zero, referencing missing files, calling invalid functions, or not handling certain input correctly.