Why Code Rusts

or Why Tests Spontanously Fail

Originally posted; 2022-02-07T16:00:00 on the TDDA Blog.

You might think that if you write a program, and don’t change anything, then come back a day later (or a decade later) and run it with the same inputs, it would produce the same output. At their core, reference tests exist because this isn’t true, and it’s useful to find out if code you wrote in the past no longer does the same thing it used to. This post collects together some of reasons the behaviour of code changes over time.¹

The Environment Has Changed

You updated your compiler/interpreter (Python/R etc.)

You updated libraries used in your code (e.g. from PyPI/CRAN).

You updated the operating system of the machine you’re running on.

Someone else updated the operating system or library/compiler etc.

Your code uses some other software on your machine (or another) machine that has been updated (e.g. a database).

Your code uses an external service whose behaviour has changed (e.g. calling a web service to get/do something).

You have updated/replaced your hardware.

You run it on different hardware (another machine or OS or OS version or under a different compiler or…)

You move the code to a different location in the file system.

E10

You have changed something in the file system that messes up the code e.g.

deleting a file the code uses

renaming a file the code uses

editing a file the code uses

emoving or renaming a directory the code uses

changing permissions on a file or directory the code uses

creating a file or directory that the code expects to create and is now unable to, e.g. because of permissions.

E11

You run as a different user.

E12

You run from a different directory while leaving the code in the same place.

E13

You run the code in a different way (e.g. from a script instead of interactively, or in a scheduler).

E14

A disk fills or some other resource becomes full or depleted.

E15

The load on the machine is higher, and the code runs out of memory or disk or some other resource; or has a subtle timing dependency or assumption that fails under load.

E15A

The load on the machine is lower, meaning part of your code runs faster, causing a race condition to behave differently. [Added 2022-02-17]

E16

The hardware has developed a fault.

E17

A systems manager has changed some limits e.g. disk quotas, allowed nice levels, a directory service, some permissions or groups…

E18

A shell variable changed, or was created or destroyed.

E19

The locale in which the machine is running changed.

E20

You changed your PYTHONPATH or equivalent.

E21

A new library that you don’t use (or didn’t think you used) has appeared in a site-packages or similar location, and was picked up by your code or something else your code uses.

E22

You updated your editor/IDE and now whenever you load a file it gets changes in some subtle way that matters (e.g. line endings, blank lines at the of files, encoding, tabs vs. spaces).

E23

The physical environment has changed in some way that affects the machine you are running on (e.g. causing it to slow down).

E24

A file has been touched² and the software determines order of processing by last update date.

E25

The code uses a password or key that is changed, expires or is revoked.

E26

The code requires network access and the network is unavailable, slow, or unreliable at the time the test is run.

E27

Almost any of the above (or below), but for a dependency of your code rather than your code itself, e.g. something in a data centre or library.

E28

Your PATH (the list of locations checks for executables) has changed, or an alias has changed so that the executable you run is different from before. [Added 2022-02-11]

E29

A different disk or share is mounted, so that even though you specify the same path, some file that you are using is different from before. [Added 2022-02-11]

E30

You run the code under a different shell or changed something in a shell startup file. [Added 2022-02-17]

Many of these are illuminated by one of my favourite quote from Beth Andres-Beck:

Mocking in unit tests makes the tests more stable because they don’t break when your code breaks.
— @bethcodes, 2020-12-29T01:26:00Z https://twitter.com/bethcodes/status/1343730015851069440

The Code Has, in Fact, Changed

You think you didn’t change the code, but actually you did.

You did change the code, but only in a way that couldn't possibly change the behaviour in the case you’re testing.

You didn’t change the code, you fixed a bug.

You didn’t change the code, but someone else did.

You didn’t change the code, but disk corruption did.

You didn’t change the code, but you did update some data it uses.

You pulled the code again from a source-code repository but

someone else had pushed a change

you checked out a different branch

you pulled from the wrong repository.

You’re on the wrong branch.

The system was restored from backup and you lost changes.

C10

You used a hard link to a file and didn’t change the file here but did change it in one of the other linked locations.

C11

You used symbolic links and though your symbolic link didn’t change, the code (or other file or files) it symbolically linked did.

C12

You used a diff tool to compare files, but a difference that does matter to your code was not detected by the diff tool (e.g. line endings or capitalization or whitespace).

C13

You are in fact running more tests than previously, or different tests from the ones you ran previously, without realising it.

C14

You reformatted your code thinking that you were only making changes to appearance.

C15

You ran a code formatter/beautifier/coding standard enforcement tool that had a bug in it and changed the meaning.

C16

You believe nothing has changed because git status tells you nothing has changed, but you are using files that aren’t tracked or are ignored.

C17

You think a file hasn’t changed because of its timestamp, but the timestamp is wrong or doesn’t mean what you think it means.

C18

A hidden file changed (e.g. a dotfile).

C19

A file that doesn’t match a glob pattern you use changed.

C20

The file is in a cloud linked folder (e.g. Dropbox) and changed remotely. [Added 2025-01-31]

C21

A coding bot changed your code [Added 2025-06-02]

Also from Beth Andres-Beck:

If you have 100% test coverage and your tests use mocks, no you don’t.
— @bethcodes, 2020-12-29T01:51:00Z https://twitter.com/bethcodes/status/1343736477839020032

You Aren’t Running the Code You Think You Are

There is another set of problems that aren’t strictly causes of code rusting, but which help to explain a set of related situations every developer has probably experienced, which all fall under the general heading of you aren't running the code you think you are.

The code you’re running is not the the version you think it is (e.g. you’re in the wrong directory).

You are running the code on a different server from the one you think you are (e.g. you haven’t realised you’re ssh’d in to a different machine or editing a file over a network).

You’re editing the code in one place but running it in another.

You have cross-mounted a file system and it’s the wrong file system or you think you are/aren’t using it when you actually aren’t/are (respectively).

Something (e.g. a browser) is caching your code (or some CSS or an image or something).

The code has in fact run correctly (tests have passed) but you’re look at the wrong output (wrong directory, wrong tab, wrong URL, wrong window, wrong machine…)

Your compiled code is out-of-sync with your source code, so you’re not running what you think you are.

You’re running (or not running) a virtual environment when you think you are not (or are), respectively.

You’re running a virtual environment and not understanding how it’s doing its magic, with the result that you’re not using the libraries/code you think you are.

M10

You use a package manager that’s installed the right libraries into a different Python (or whatever) from the one you think it has.³

M11

You think you haven’t changed the code/libraries/Python you’re using, but in fact you did when you updated (what you thought was) a different virtual (or non-virtual) environment.

M12

You have a conflict between different import directories (e.g. a local site-packages and a system site-packages), with different versions of the same library, and aren’t importing the one you think you are.

M13

You think the code hasn’t changed because you recorded the version number, but there was a code change that didn’t cause the version number to be changed, or the code has multiple version numbers, or the code is reporting its version number wrongly, or the version number actually refers to a number of slightly different builds that are supposed to have the same behaviour, but don’t.

M14

You have defined the same class or method or function or variable more than once in a language that doesn’t mind such things, and are looking at (and possibly) editing a copy of the relevant function/callable/object that is masked by the later definition. [Added 2022-09-14]

M15

A web server or application server has your code in memory and changing or recompiling your code won’t have any effect until you restart at web server or application server. This is really a variation of M5, but is subtly different because you wouldn’t normally think of this as caching. [Added 2024-03-30]

These are the ones that make you question your sanity.
TIP If what’s happening can't be happening, trying introducing a clear syntax error or debug statement or some other change you should be able to see. Then check that it shows up as expected when you’re running your code.
Almost every time I think I'm losing my mind when coding, it's because I'm editing and running different code (or viewing results from different code).

Time has Moved On

T1 Your code has a (usually implicit) date/time dependence in it, e.g.

it uses 2-digit dates

it assumes it’s running in 2022

it assumes it’s not 29th February, or 1st January, or isn’t a weekend…

it assumes something else that’s not true about (computer) time (no leap seconds, no daylight savings times, no time-zones, no half-hour-aligned timezones…)

it uses 2-digit dates with a pivot year and time (or some computed time the code uses) moves past the pivot year.

T2 Time is ‘bigger’ in some material way that causes a problem, e.g.

Y2K

Unix 2038 (when the number of seconds from 1 Jan 1970 overflows signed 32-bit integers)

Number of days since the code was written needs more digits (10, 100, 1000).

While the code is running, daylight savings time starts or stops, and a measured (local) time interval goes negative.

Your code uses Excel to interpret data and today’s a special date that Excel doesn’t (or more likely does) recognize.

The system clock is wrong (perhaps badly wrong); or the system clock was wrong when you ran it before and is now right.

Resources Used by the Code Have Changed

A resource your code uses (a database, a reference file, a page on the internet, a web service) returns different data from the data it always previously returned.

A resource your code uses returns data in a different format e.g. a different text encoding, different precision, different line endings (Unix vs. PC vs. Mac), presence or absence of a byte-order marker (BOM) in UTF-8, presence of new characters in Unicode, different normalization of unicode, indented or unindented JSON/XML, different sort order etc.

A resource you depend on returns “the same” data as expected but something about the interaction is different, e.g. a different status code or some extra data you can ignore, or some redundant data you use has been removed.

Stochastic and Indeterminate Effects

Your code uses random numbers and doesn’t fix the seed.

Your code uses random numbers and does fix the main seed but not other seeds that get used (e.g. the the seed for numpy is different from Python’s main seed).

A cosmic ray hits the machine and causes a bit flip.

The code is running on a GPU (or even CPU) that does not, in fact, always produce the same answer (order of execution).

The code is running on a parallel, distributed, or multi-threaded system and there is inderminacy, a race condition, possible deadlock or livelock, or any number of other things that might cause indeterminate behaviour.

Your code assumes something is deterministic or has specified behaviour that is in fact not determinisic or specified, especially if that result is the same most but not all of the time, e.g. tie-breaking in sorts, order of extraction from sets or (unordered) dictionaries, or the order in which results arrive from asynchronous calls.⁴

Your code relies on something likely but not certain, e.g. that two randomly-generated, fairly long IDs will be different from each other.

Your code uses random numbers and does fix the main seed, but the sequence of random numbers has changed. This has happened with NumPy, where they realised that one of the sampling functions was drawing unnecessary samples from the PRNG. In making the sampler more efficient, they changed the samples that were returned for the same PRNG seed.
[Contributed by Rob Moss (@rob_models and @rob_models@mas.to), who “had a quick search for the relevant issue/changelog item, but it was a long time ago (~NumPy 1.7, maybe).” He “couldn’t find the original NumPy issue, but here’s a similar one: https://github.com/numpy/numpy/issues/14522”. Thanks, Rob!]

It Never Worked (or didn’t work when you thought it did)

[Added 2024-07-19]

I realised there’s another whole class of errors of process/errors of interpretation that could lead us to think that code has “rusted” despite not having been changed. These are all broadly the same as one of the explanations offered before, but now for the original run when you thought it worked, rather than for the current or new run, when it fails.

You thought you ran the code before, and that it worked correctly, but you are mistaken: you didn’t run it at all, or it in fact failed but you did not notice.

You did run the code before, but picked up the output from a previous state, before you broke it, when it did work.

You did run the code before, and it did produce the wrong output then as now, but you used a defective procedure or tool to examine the output then, and failed to realise it was wrong/failing.

You did run the code before, and it did pass, but you passed the wrong parameters/inputs/whatever and are now passing the correct (or different) parameters/inputs/whatever so it now fails as it would have done then if you had done the same.

¹ If you have think of other reasons code rusts, do let me know and I’ll be happy to expand this list (and attribute, of course)

² Touching a file (the unix touch command) updates the last update date on a file without changing its contents.

³ For this reason, a lot of people prefer to run python -m pip rather than pip, because this way you can have greater confidence that the module is getting installed in the site-packages for the version of python you’re actually running.

⁴ Most of these kinds of indeterminacy will, in fact, usually be stable given identical inputs on the same machine running the same software, but it can take very little to change that, and should not be relied upon. Why Code Rusts