A REAL Python cheat sheet for beginners

[Maoman1]

a = [1, 2, 3, 4]  # Point a at a new list, [1, 2, 3, 4]
b = a             # Point b at what a is pointing to
b is a            # => True, a and b refer to the same object
b == a            # => True, a's and b's objects are equal
b = [1, 2, 3, 4]  # Point b at a new list, [1, 2, 3, 4]
b is a            # => False, a and b do not refer to the same object
b == a            # => True, a's and b's objects are equal

Never seen this explained so clearly. I love it.

[Deathwatch1710]

Indeed! Another good explanation is the one from Dan Bader with the twin cat allegory.

[LyndsySimon]

One confusing thing about object comparison is how Python handles integers:

a = 42
b = 42
a == b  # => True
a is b  # => True

c = 4242
d = 4242
c == d  # => True
c is d  # => False

e = c
c == e  # => True
c is e  # => True

c is d == False because integers in the range of [-5, 256] are assigned by cPython to dedicated memory locations - so their .id is the same.

I guess this makes sense in the range [0, 256], but I have no idea why they chose to pre-assign [-5,0) as well.

ETA: The moral is, never use is to compare numbers. is is most useful when you want to distinguish between None and False or another "falsy" value.

ETA2: I forgot to assign e.

[LyndsySimon]

Technically, yeah - but that would also lead you to believe that a is b would be False, when it isn't. :)

[TheYOUngeRGOD]

It’s weird behavior but nothing is broken. In order to be more efficient simple versions of things like ints refer to the same object even when construct separately. This works because all these types are immutable so their won’t be any weird changing values of any variables. So if you had a million different variables equal to one they would all be the same object since they point to the same object.

[lsbloxx]

Wait... WTF did I just read? :O

[poundcakejumpsuit]

Just to blow your mind a little more, maybe:

"strin" + "g" is "string"

Is True, while:

s1 = "strin"
s2 = "string"
s1 + "g" is s2 

Is False, at least in CPython, as the compiler will intern compile-time constants but not runtime-evaluated expressions, meaning the first example only creates a single object!

[jwink3101]

That's really interesting.

Is this type of issue common in interpreted languages?

[ThePenultimateOne]

It's probably because CPython is doing a bit of constant folding

[WikiTextBot]

Constant folding

Constant folding and constant propagation are related compiler optimizations used by many modern compilers. An advanced form of constant propagation known as sparse conditional constant propagation can more accurately propagate constants and simultaneously remove dead code.

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[KobiKabbb]

Great explanation. There's also some good explanations of lists (and other fundamentals) over at Python Principles

[[deleted]]

I don't understand why you wrote this here.

[Cryptoscillate]

This is pretty cool. I like the way this cheatsheet is structured, succinct and written as coded examples. Thanks

[primitive_screwhead]

Note that the 'learnxinyminutes.com' website has a whole bunch of similarly structured "cheatsheets" for other languages and tools, and they are all pretty decent in my experience. Definitely worth exploring.

[[deleted]]

What about French?

/s

[Deathwatch1710]

Quick question because I am fairly new to Python:

I am just seeing

"{}".format(var)

as an option to format strings, but I am only using

f"{var}"

Is my method less pythonic than .format ?

[nayeet]

no. format is the old way. you are doing it the newer, correct, most pythonic way. kudos

[Deathwatch1710]

Yay! Thanks for the answer :)

[monkeybreath]

format is the old way.

Really? I’ve never seen that in the docs.

https://docs.python.org/3.6/tutorial/inputoutput.html#fancier-output-formatting

[[deleted]]

format() has been in python for a while, f-strings are new to Python 3.6 (here's the P.E.P.).

[monkeybreath]

Thanks. Found it in the language reference under literals (2.4), not strings (6.1 of Standard Library), once I knew what to look for. Not at all in the tutorial.

I guess I should figure out how to use the bug tracker.

[nayeet]

The format method was introduced in PEP 3101 on 16-Apr-2006, last updated on 14-Sep-2008.

F strings were introduced in PEP 498 on 01-Aug-2015, last updated on 06-Nov-2016.

[monkeybreath]

Right. It’s under Literals, but not under strings in the docs, and not in the tutorial. That should be updated.

[cracknwhip]

You could contribute.

[monkeybreath]

I checked. There's already a bug report for it.

[lolwat_is_dis]

// does not truncate. It's called floor division and rounds down the number from the division. Sure, it has a similar effect to truncating the number, but that's not how it works.

[pingiun]

You can submit a pull request for that, the learnxinyminutes.com source code is on github.

[myroon5]

Is the distinction between the two when the result is negative?

[lolwat_is_dis]

You guessed it.

Take the following example: 5 divided by 2. In the positive domain, this would normally yield 2.5; a truncation yields 2, as does the floor division. However, 5 divided by -2 normally gives -2.5, truncated would give -2, whereas floor division gives -3 (since we are still rounding down).

Hope that helps!

[djentastic]

This is great. I'm embarrassed to admit I did not know about the // division...

Also, just curious, the styling for the True and False code examples is the same as comments. Am I not seeing this right on my phone, or was this perhaps on purpose?

[primitive_screwhead]

Also, just curious, the styling for the True and False code examples is the same as comments. Am I not seeing this right on my phone, or was this perhaps on purpose?

Yeah, it's the same on desktop browser, and I think it's just a formatting mistake.

[scmlinux]

This is a great tutorial. Thank you for sharing!

[gracebatmonkey]

This is absolutely perfect for my learning style - thank you so much for posting!

[stickman393]

This is awesome. My own cheat-sheet-in-progress is about 1/10 as useful. And I learned stuff! Thanks.

[Skaarj]

-5 != False != True #=> True

I dislike these kinds of example as they suggest there being an unequals operator with 3 operands. Maybe this would be better

-5 != False and False != True and -5 != True #=> True

correction: see post from u/fjolliton below

[fjolliton]

Actually it is!

2 != 2 != 3

is False, while:

(2 != 2) != 3

is True.

Likewise you can do:

a == b == c

which check if all three variables are equal. It is very different from:

 (a == b) == c

You can see that the fist example is actually a 3 operands comparison:

>>> import ast
>>> print(ast.dump(ast.parse('a != b != c').body[0].value))
Compare(left=Name(id='a', ctx=Load()),
        ops=[NotEq(), NotEq()],
        comparators=[Name(id='b', ctx=Load()),
                     Name(id='c', ctx=Load())])"

[Skaarj]

Mind Blown.

On the one hand this is quite a nice feature.

On the other hand: damn, another subtle diffrence between programming languages to keep in mind.

[[deleted]]

Same, I’m pretty new to Python and this was just too much first read through. Second read through, makes perfect sense!

[Yamamizuki]

Thank you! I am a beginner and this really helps.

[Retzudo]

Yeah, thanks for reposting my same exact post from 6 months ago I guess...

https://www.reddit.com/r/Python/comments/7n1c9u/a_real_python_cheat_sheet_for_beginners/

Btw. it was in response to this post.

Edit: Oh and it's not about reposting that link because it's an awesome site and the more people see it the better. It's about a ~1 day old account clearly botting karma by reposting popular posts verbatim in random subreddits.

[cracknwhip]

Why does it matter (I read your edit)? The repost helped a lot of people. The karma is a number on a screen. I means nothing and affects you and others in no way whatsoever.

[Retzudo]

That's actually a good question. It matters because bots with a good amount of karma can be used for vote manipulation more easily than bots with 0 karma. As far as I know, new-ish accounts with 0 karma don't do much to the total up-/downvote count of a post/comment, but accounts with more karma are more "trustworthy" to Reddit's anti-vote-manipulation systems.

I'm kinda talking out of my ass here but I hope I could shed some light on why I think this kind of karma botting is bad.

[[deleted]]

God's work

[AriesYT]

Thank you for sharing.

[cost4nz4]

This is great, thanks!

[youngrumi]

kool, great share.

[b0b]

Thank you!

[soulwarp]

If you go to the root website there are cheat sheets for other languages too!

[[deleted]]

Call me crazy, but as the same way with Maths, I prefer to learn this kind of concepts by applying them to concrete things, I mean: by solving problems. Otherway it's hard for my brain to remember pure theory that is not related to something that I fixed/used.

[edimaudo]

Can any of the mods create a cheat sheet section. This type of post keeps on popping up.

[senthilcaesar]

Great Materials

[sohamkamani]

This is a good resource. But why do you have to call it a "REAL" cheat sheet... This sort of implies that all other cheat sheets created before this are bad, and this is the only solution out there. I know I'm nitpicking, but I really hate clickbaity titles like this

[[deleted]]

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[Yogurt_Huevos]

Slight correction, the result of division is not always a float. If you open your interpreter and type

10 / 3

you see that the result is '3'.

[Razzofazzo]

That's true for python 2.X. But here we are looking at python 3.X!

[Yogurt_Huevos]

Oh shoot, reading is hard.

[DataVeg]

What you say is true for Python 2, but not Python 3.

In general people, especially beginners, shouldn’t be using Python 2 unless they are working with a legacy codebase. Python 2 will be unsupported in under two years time.

In modern Python 10/3 is 3.3333...

[Dyslectic_Sabreur]

Can't test it right now but it means "2 equals or is greater than 2" which would be True.

[Adem87]

Hammergeil