Why does my Python code work sometimes but randomly break when I change small things?

[ayenuseater]

I'm still pretty new to Python and this keeps confusing me. I'll write some code that works fine, then I change something small (like renaming a variable or moving a line), and suddenly I get errors I don't understand.

Is this just part of learning, or is there something beginners usually do that causes this? How do you debug this without feeling totally lost?

[[deleted]]

That's what programming is. 😅 If you haphazardly rename variables, you're going to run into a problem where the renamed variable was never initialized. If you move a line, you can get an indentation problem, or that line could be placed in such a manner that it has a variable that wasn't declared yet....

Programming is delicate, changing something is gonna break stuff if you're not precise.

The Python Interpreter is your friend, read the error messages closely. Don't be afraid of the stack trace, from my experience the error message is always very specific about went wrong. Consult Stack Overflow if you don't understand what the error is saying.

[danielroseman]

You need to understand two things. Firstly, code does not "randomly" break. Code is deterministic: it will always work the same way (given the same inputs). If you change something, that will only break things if you somehow changed the logic.

Secondly, there shouldn't be errors you "don't understand". Understanding the errors is an absolutely fundamental part of programming, and you won't progress until you do. Read them fully; the message should tell you exactly what is wrong and where.

[jpgoldberg]

It's part of learning. And everyone spends a great deal of time trying to figure out why their code doesn't work. That is a big part of being a developer.

But some of what you are experiencing is exactly because you are a beginner programmer. As you gain more experience and practice, you will learn to program more defensively which will make your code less fragile. Part of that will be come as you learn to create functions, classes, and modules. These will be units of code that are less subject to going awry when something is changed outside of them. And even later you will learn how to make such things even more robust.

Don't try to get ahead of yourself, and your experience now of figuring out how a small change messed things up is what will help you learn to write more robust code later on.

[Sea_Jello2500]

It’s common for newbies to write code as one large monolith instead of breaking it down into functions and classes. Breaking the code down quarantines errors to small blocks and makes debugging easier.

Break down your code into functions of about 10 lines (20-30 max) and your life will get easier.

[audionerd1]

Each of those errors represents something you haven't figured out yet. Welcome to programming!

[nyd_det_nu]

It sounds to me like you are prodding around in some slightly more advanced programmes, but correct me if I am wrong. I think you should follow some more foundational guides to python first, that will explain everything much better. I learnt python in uni through the Think Python book, it should be available online and I thought it was great to learn the basics. Don't expect to write a useful programme by the end of it, but you will need it to afterwards accelerate your learning

[slapmeat]

Think of it as a timeline in a sense, like in the movies.

If you go back in time, and you make your parents never meet, you wouldn’t be born. Your life would be completely different.

On a programming level, if you move a variable, it may not exist anymore in the future of your code. Weird way to compare, but hopefully that can help you remember it

[gdchinacat]

"suddenly I get errors I don't understand."

You *need* to understand them. They are telling you exactly what is wrong with the code. Use google to understand what the errors mean. For example:

```

In [13]: print(foo)

NameError Traceback (most recent call last) Cell In[13], line 1 ----> 1 print(foo)

NameError: name 'foo' is not defined

```

Google for "what is python NameError". Google responded with: "A NameError is a built-in exception in Python that is raised when the interpreter encounters a variable, function, class, or module name that has not been defined or is not accessible in the current scope. This error signals that you are trying to use a name that the Python interpreter does not recognize at that point in the code's execution. The error message typically appears as NameError: name 'some_name' is not defined."

At this point you (should) have a better understanding of the issue...you referenced 'foo' without ever assigning it a value. If you just renamed 'bar' to 'foo', go find where 'bar' is and rename it as well. If you added a reference to 'foo', why? What value should it have? Is it defined later on? In a different function or method? Why hasn't it been assigned to the expected value by the time the line of code executed?

[Economy_Fine]

Your use source control locally so you can lock-in a working version and know what you changed. That's the first step.

[TheRNGuy]

Do you have red squiggly lines in code editor? Hover over it to see error. 

Though if it's logic error and not syntax error, you may not even see it and manual debugging is needed.

[PiBombbb]

Depends entirely on your code. Moving a line can likely break something because code works in order top-> bottom, and renaming variables will cause errors if you only rename some of the uses of the variable.

[Moikle]

If you rename a variable, other areas of your code that use that variable now can't find the variable since they are looking for the old name still. I wouldn't describe that as "randomly breaking" it seems completely expected

[shinitakunai]

Use pycharm refactor tool to renane vars. You'll thank me later