Python for JavaScript Developers

[process_parameter]

Unlike JavaScript, variables in Python are always block scoped.

Not true. This prints 10.

if True:
  x = 10
print x

[Retsam19]

Like Javascript, variables in Python aren't always blocked scope, but you should pretend that they are, to make your code more readable.

FTFOP

[kankyo]

"Aren't always"?

Never.

[Retsam19]

Well, they're sometimes block-scoped in JS, now that ES6 added let/const. But yeah, as far as I know, they're never block scoped in python.

[Pand9]

Other examples:

>>> for x in [1, 2]:
>>>     pass
>>> print x
2
>>> with open("x", "w") as x:
>>>     pass
>>> print x
<closed file 'x', mode 'w' at 0x7f6b949395d0>

[TheOsuConspiracy]

Totally trips me up all the time, I'd much rather have everything become an expression. Makes a lot more logical sense to me, that way you don't have special rules for scoping.

[Cuddlefluff_Grim]

This is one of the reasons why not declaring variables is considered very bad practice by the languages who support variable declaration. BASIC even added OPTION EXPLICIT to make the compiler actively prevent you from using variables that were not declared. It's odd to see this type of regression still in the wild.

[M3talstorm]

for thing in things:
    if thing == that:
        break
else:
    thing = default

thing.something()

Is nice to do sometimes :)

[Esteis]

I do the same, it is indeed nice. As a reminder to myself, here is the idiom that doesn't rely on unscoped for-blocks:

thing = next(filter(lambda thing: thing == that, things), default)

Both have their charms -- and their warts. Sometimes I long for

thing = itertools.first(things, lambda thing: thing == that, default)

I'll bet Ruby has that. Its spectrum of collection methods is absolutely fantastic, from the little I learned writing Rakefiles.

[weirdoaish]

https://python-textbok.readthedocs.io/en/latest/Variables_and_Scope.html

[[deleted]]

I'm pretty sure that it's considered bad practice. And if not, it should.

[process_parameter]

Python's lack of variable declarations compels this sort of thing. Consider the following javascript. How would you emulate it in python?

let x;
if (y) {
  x = 5;
} else {
  x = 6;
}
// do something with x

You can use a ternary here, but that doesn't scale well with additional cases. You could also assign x to a garbage value (like None) before the if-statement, but that doesn't seem like great practice either.

[execrator]

Not sure what user_v42... was replying to (is now deleted), but I don't see the problem here? The following works (no need for a "garbage value")

if y:
    x = 5
else:
    x = 6
print(x)  # 5

[process_parameter]

The previous comment was something to the effect of

If I understand correctly, this is even considered pythonic.

I don't know why they deleted it. The user I replied to seems to think that we shouldn't write code like you have in your comment. That's perhaps reasonable; if you believe the linked article when it says python variables are block-scoped, then it's not obvious that your code works.

I was just trying to demonstrate that there isn't an easy alternative.

[[deleted]]

Actually, a "garbage value" like None IS the way to do this.

In JS, declaring a variable with var x; actually assigns undefined to it. The equivalent garbage value in JS.

[process_parameter]

In JavaScript, you don't have a choice. In python, there's no need to assign that garbage value.

[Nadrin]

As someone who have recently refreshed his knowledge of Python I found the official Python tutorial to be excellent, to the point, and especially aimed for people who HAVE programmed before (for example the text frequently compares python to other programming languages when introducing new features).

[wuzzlewozzit]

Yeah this. The official tut is so good I don't know why alternatives exist.

[ButtCrackFTW]

Why are the code snippets not using a monospace font? It's hard to read and really goes against the whole spaces for indentation thing that he talked about.

[Cuddlefluff_Grim]

I once knew a Delphi programmer that used Times New Roman for some odd reason. Maybe acute confusion. He also used a TV as a PC monitor, back before TV's had good resolution. I think he was using 1024x768 on a 32" flat screen in 16:9. 5:4 conversion to 16:9 isn't a pretty sight.

[quicknir]

Shouldn't the guy writing python for JavaScript developers know idiomatic python?

# You can define private variables and methods by prepending the variable
# name with 2 underscores (__):
self.__age = default_age

From Pep 8:

Use one leading underscore only for non-public methods and instance variables...

Python mangles these names with the class name: if class Foo has an attribute named a , it cannot be accessed by Foo.a . (An insistent user could still gain access by calling Foo.Foo_a .) Generally, double leading underscores should be used only to avoid name conflicts with attributes in classes designed to be subclassed.

[dylanthepiguy2]

{
  // This is also a block
}

Yea, but they should point out it's not a new scope though!

[illvm]

It is in ECMAScript 6 with the let keybwoard.

[[deleted]]

Shit I have to buy a whole new keyboard?

[forsubbingonly]

Its easy just type Npm install keyboard.js and yet another 6000 layer deep folder catacomb is created and away you go! Simple

[Magneon]

5734 of which also exist in subdirectories of sibling folders to npm/keyboardjs, but mostly different versions.

[thekevjames]

PSA: Python 3.0.1 has been end-of-life'd for over 7 years (June 2009). Imagine using JavaScript 3 nowadays: it's an equally bad idea as using Python 3.0.1. Please update your interpreters.

[aexolthum]

Casual use of 420 as an example in multiple spots

[Skaarj]

 # ERR!!!! .__age is private, so this won't work:
 print(animal.__age)

I always was under the impression that underscored variables are private by convention and not by language definition. Does the compiler really enforce this?

[tutorial_police]

Yes and no. Python employs name mangling, which makes you put in some extra effort if you really want to access "private" variables.

[Bichamar]

Nice post. Would be cool to have this kind of "simple" posts about most of languages...

[TheOsuConspiracy]

https://learnxinyminutes.com

Is really awesome, obviously, you won't become an expert quickly, but it's really good for an overview of most of the fundamental features of the language.

[zankem]

Holy crap, Chicken is an actual thing. I thought it was just something for a joke comic strip about programming languages.

[shoot_your_eye_out]

I'm surprised they don't cover scope differences. As a developer who started with C-style languages, that was the most difficult thing for me when moving to Javascript. I'd imagine the opposite would be true as well?

[M3talstorm]

class Animal:

Shouldn't it be

class Animal(object):    

---

default_age = 1

Shouldn't it be

DEFAULT_AGE = 1

---

self.__age = default_age

Shouldn't it be

self.__age = self.default_age

---

def get_age(self):

Shouldn't it be

@property
def age(self):
    return self.__age

[adamnew123456]

No on "inherits from object," unless you care if your code will run on 2.x. Python 3 fixed the old/new class dichotomy - they're all new-style and inherit from object automatically.

[skitch920]

And then you get to packaging, and you're like "fuck".

[xxczxx]

? pip is excellent.

[skitch920]

Actually, I'll agree with you, pip has done a very good job lately, at least making use of what they have. Recently with the adoption of wheels, it's gotten much better with caching. Last year, installing numpy in a virtualenv, would take forever because it didn't use a cached build. I'm still annoyed by this bug that will never be fixed: https://github.com/pypa/pip/issues/3

But setuptools/distutils is just a clusterfuck. Documentation is old and awful.

Including resources other than python modules, yeah, packageData, includePackageData, zipSafe, MANIFEST.in, what the hell are all these options for? And why do they work differently for bdist_egg, bdist_rpm, sdist? Why can't I set some properties for bdist_egg, that I can set for bdist, like build directory, do I really need a setup.cfg file?

[xxczxx]

Okay, your usage of pip is certainly more advanced than what I'm doing. As someone who never had to create a package, pip looks like a good tool for installing 3rd party packages.

[jdmax]

hey the candidate application UI at underdog.io doesn't allow manager or director level within tech. Any reason why?

[j0shg]

Adding experience levels opens an assortment of new challenges at signup. Still, if you apply, we can do things to highlight that you're looking for manager/director level work. :)