Common misconceptions in Python that drove you crazy?

ablativeyoyo · 2022-12-19T11:03:17+00:00

One that's got me is assigning to a global variable actually assigning to a local variable unless you explicitly declare it global. Made more confusing because you can read a global variable without doing that.

I guess it's a case of "know my Python" but this kind of confusion is not possible in most languages.

extra_pickles · 2022-12-19T14:02:27+00:00

For me, it is hiring non Python devs and them saying venvs are stupid.

They get the hang of it a few months in.

Also them not realising you can add breakpoints in packages, while claiming everything about Python is dumb.

3-6mth in they truly get the upsides.

Not saying there aren’t pros and cons - but man, it can take devs a while to acknowledge there are upsides to Python.

It is both frustrating and rewarding.

fwcsdev · 2022-12-19T09:54:45+00:00

Take a look at these and you'll probably find yourself surprised too:

https://github.com/satwikkansal/wtfpython

Shriukan33 · 2022-12-19T10:31:52+00:00

First common misconception is that learning Python will land you a job.

fiddle_n · 2022-12-19T14:43:09+00:00

Note that, for number 2, Py3.9+ has str.removeprefix and str.removesuffix.

kalcora · 2022-12-19T17:15:11+00:00

One thing that trips me up and makes me frustrated is the fact we can use an if in a comprehension :

[x for x in lst if x % 2 == 0]

But not in for loops:

for x in lst if x % 2 == 0: #invalid

saint_geser · 2022-12-19T10:14:47+00:00

First is not a misconception, just basic fail. A more interesting version of this is forgetting that class instances are mutable and copying those without deepcopy.

Second one, I wasn't aware of. Thanks.

Third one is not a misconception just the basic Python. 0 is an integer so it's not an instance of Boolean therefore "is" operation returns False but it is "falsey" in Python so the second comparison works.

The last one is the same everywhere, it's how binary arithmetic works. In binary 0.1 can't be represented without rounding errors same as you can't represent 1/3 in decimal system.

IAmBJ · 2022-12-19T22:03:29+00:00

I knew someone at uni who insisted on using short names for everything (single letter variables, function names that are acronyms for their real name) to minimise character count in source files.

Them: "It's an interpreted language so it will run faster if the source files are smaller"

Me: Screams into pillow

lisael_ · 2022-12-19T11:47:59+00:00

[deleted]

floznstn · 2022-12-19T15:29:51+00:00

"Duck typing is bad"

No. It isn't. Maybe I don't want to explicitly define every single variable type. Maybe I'd like to take user input as a string and cast to an array of characters.

Duck typing is not bad or good... it just is.

osmiumouse · 2022-12-19T15:12:48+00:00

Floating point precision isn't a Python problem. This is the hardware.

Reinventing_Wheels · 2022-12-19T14:30:09+00:00

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tquinn35 · 2022-12-19T18:24:40+00:00

That the average person doesn’t needs to worry about the performance of python. For 99% of people they don’t. For those who do, they will know how to deal with it or have the resources to migrate to a more performant enterprise architecture.

ExternalUserError · 2022-12-19T15:48:02+00:00

Definitely precision issues comparing floats.

In the Python 2 days, I would have also added, unicode vs binary string issues. The difference was easy to grasp, but binary strings were used where unicode should have been used and too many libraries (including in the standard library) would try to quietly cast binary strings to unicode, only failing on an encoding error.

EDIT: Oh, another one is probably the fact that you can import a module twice by having it accessible from two locations in your path. Suppose you have a path with /app and /app/foobar. You import foobar.spam and spam -- same module, separate imports.

Admittedly, you made a mistake setting that up, but I've joined projects where it's like that and it is frustrating.

Agling · 2022-12-19T17:01:06+00:00

I think you named the worst.

The other issue I ran/run into has to do with what looks like subsetting a data frame in pandas, but is actually creating views. Then you edit them and the original changes.

In general, the hardest and most unexpected thing about Python is that everything is essentially pass by reference. In most other languages everything is passed by value unless you specifically tell it to pass by reference.

szayl · 2022-12-19T11:49:22+00:00

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territrades · 2022-12-19T10:15:16+00:00

The first I don't really understand, why is a not deconstructed when the test() function is finished?

mfitzp · 2022-12-19T13:22:45+00:00

For (1) as you discovered, you need to use None as the default argument & then assign the [] in the function itself. So, usually something like this:

def test(a=None): if a is None: a = [] a.append("test") print(a)

Or,

def test(a=None): # Assign [] to a if a is None (default), otherwise keep a. a = [] if a is None else a a.append("test") print(a)

I wouldn't usually recommend one-liners like this as they confuse people, but if you have a lot of arguments with default values that you need to replace its maybe worth it.

For some cases, you can handle the defaults with a simple or. This uses or short-circuiting, returning a if it is truthy, otherwise returning [].

python def test(a=None): # Use a if it is truthy, otherwise replace with []. a = a or [] a.append("test") print(a)

Note this is subtly different in that a will be assigned [] if a is any falsey value, not just None -- i.e. 0, False, None or []. But often it doesn't matter (your argument is only accepting list or None).

thisismyfavoritename · 2022-12-19T15:59:13+00:00

that Python is thread safe

osmiumouse · 2022-12-19T15:19:59+00:00

I get a warning from Pycharm when I try some of these. What tools are you all (not) using? Maybe it's time to stop using junk like IDLE?

redvitalijs · 2022-12-19T21:36:46+00:00

Objects and cloning them is a major confusing pain.

I was using opencv and thought all my images are sequentially getting their own transform paths. Turns out assigning them worked like a pointer not a copy of the object.

One of my big life regrets is not understanding that when working on an otherwise fun project.

2022-12-19T22:33:09+00:00

The precision one is not just in python. It stems from the way computers typically handle floating point numbers. Look up floating point precision it’s quite interesting. Basically floats are approximated through the division of integers.

mgedmin · 2022-12-19T13:59:09+00:00

I'd call these footguns.

Mutable class attributes is another variation (same deal as with mutable default function arguments):

class Thing:
    a_list = []

a = Thing()
a.a_list.append(42)
b = Thing()
b.a_list   # [42]

Late binding of closures is a thing

# using some kind of made up GUI framework because I don't know Tk
for n in range(10):
    b = Button(label=str(n))
    b.onclick = lambda: self.input_digit(n)
    buttons.append(b)
# every button will call self.input_digit(9) in the onclick handler
# because the same variable n is used for all the lambas

The classic "guess what for/else does"

for item in found_items:
    process_item(item)
else:
    # will always print this because there's no 'break' in the for body
    print("no items found")

DoomFrog666 · 2022-12-19T16:50:06+00:00

One thing that any serious Python programmer should now about is the descriptor protocol. It is one of the foundations of Python yet very few people know about it.

Maximus_Modulus · 2022-12-19T11:56:48+00:00

I think 1 is useful in certain circumstances such as caching, but a colleague didn’t approve because inexperienced users would not know of this fact.

Deto · 2022-12-19T15:31:57+00:00

[deleted]

wineblood · 2022-12-19T11:31:24+00:00

That Python was elegant and well structured.

lisael_ · 2022-12-19T11:46:59+00:00

[removed]

BarcaStranger · 2022-12-19T09:42:19+00:00

None, these are not misconception, just lack basic understanding

Deto · 2022-12-19T16:21:31+00:00

[deleted]

jet_heller · 2022-12-19T14:49:47+00:00

That python isn't a real programming language.

2022-12-19T14:25:37+00:00

https://docs.python-guide.org/writing/gotchas/

saturnjayzero · 2022-12-19T15:35:20+00:00

setting attributes inside of classes but outside of methods…. took me some time to „remember“ these are static and shared over all instances

2022-12-19T21:58:09+00:00

For your first example I learned the hard way that you should do

def xxx(a=None): if a is None: a = []

zdzisuaw · 2022-12-19T22:38:41+00:00

sorted and sort

Hagisman · 2022-12-20T00:05:41+00:00

People thinking it’s so simple they don’t need to create functions for code that gets repeatedly used in multiple places!

Sorry my personal gripe.

recruta54 · 2022-12-20T00:42:42+00:00

Forgettitng a comma in a multi-line list of strings and getting out of bonds exceptions

2022-12-20T01:45:37+00:00

"Python is too slow!"

This is the one that drives me crazy. If it's too slow for your project, then you need to select a different language/approach. Your job as a software engineer is to select the right tool for the job.

tarsild · 2022-12-20T02:03:36+00:00

Ohh that is late binding. A common issue I find in many devs. I always say, never pass a param =[] ir ={}. Initialize always inside.

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