There are a lot of questions on StackOverflow asking "what's the deal with self?"

Many of them are asking a language-design question: Why does Python require explicit self when other languages like C++ and friends (including Java), JavaScript, etc.

1

Tkinter makes slapping together a simple GUI very easy. But unfortunately, many of its features aren't very well documented.

7

In Python 2.x Tkinter code, you see a lot of stuff like this:

class MyFrame(Frame): def __init__(self, parent, n): Frame.__init__(self, parent) self.n = n Why?

Inheritance and overriding

Some people start on Tkinter before getting far enough into learning Python.

1

Does Python pass by value, or pass by reference?

Neither.

If you twist around how you interpret the terms, you can call it either. Java calls the same evaluation strategy "pass by value", while Ruby calls it "pass by reference".

9

In database apps, you often want to create tables, views, and indices only if they don't already exist, so they do the setup work the first time, but don't blow away all of your data every subsequent time. So SQL has a special "IF NOT EXISTS" clause you can add to the various CREATE statements.

November 7th, 2013

Sometimes you want to write a round-trippable __repr__ method--that is, you want the string representation to be valid Python code that generates an equivalent object to the one you started with.

First, ask yourself whether you really want this.

Many novices notice that, for many types, repr and eval are perfect opposites, and assume that this is a great way to serialize their data:

def save(path, *things): with open(path, 'w') as f: for thing in things: f.write(repr(thing) + '\n') def load(path): with open(path) as f: return [eval(line) f

1

There are a number of blogs out there that tackle the problems of callbacks for servers, or for Javascript, but novices trying to write Python GUIs shouldn't have to learn about the different issues involved in servers, or a whole different language.

Imagine a simple Tkinter app.

21

So you've installed Python from an official binary installer on python.org's Releases page, you've installed Xcode from the App Store and the Command Line Tools from Xcode, you've installed pip from its setup script.

Stack Overflow is full of questions where the answer is to create a "multidict", a dict mapping each key to a list of values.

There are two ways to do this, using defaultdict, or using a regular dict with setdefault.

1

The problem

Often, using an iterator lazily is better than generating a sequence (like the one you get from a list comprehension).

2

A lot of people—not just novices—mix up parameters and arguments, especially when it comes to things like how default-valued parameters and keyword arguments, or argument unpacking and variable parameters.

3

How grouper works

A very common question on StackOverflow is: "How do I split a sequence into evenly-sized chunks?"

If it's actually a sequence, rather than an arbitrary iterable, you can do this with slicing.

1

This question comes up over and over and over on Stack Overflow—even if it's not the main question itself (as in those examples), it's often a side argument in the comments.

2

Let's say you have a network server built around a single-threaded event loop—gevent, Twisted, asyncore, something simple you built on top of select.select, whatever. It's important to handle every event quickly.

People want sorted collections in the stdlib, but no one has ever made a solid proposal to add them.

4

You're writing a GUI app using Tkinter or PySide or your favorite GUI library, and testing it in-place, everything works.

Then you build a .app bundle out of it, double-click it in Finder, and it can't read your environment variables.

There's a pair of intertwined threads in python-ideas about coming up with a way to break out of list comprehensions and generator expressions early.

Oscar Benjamin gives a good example:

isprime = all(n % p for p in takewhile(lambda p: p**2 < n, primes_seen))

That's ugly.

4

What's the difference between a list comprehension, and calling list on a generator expression? (By the way, everything below applies to set comprehensions and, with trivial tweaks, dict comprehensions, but I'm only going to talk about lists for simplicity.)

To be concrete, what's the difference be

Recently, there have been a few proposals to change Python's syntax to make it easier to avoid break and continue statements.

The reasoning seems to be that many people are taught never to use break and continue, or to only have a single break in any loop.

1

Sometimes, you need to split a program into two parts.

Look at this familiar code:

class Foo(object): def __init__(self, a): self.a = a def bar(self, b): return self.a + b foo = Foo(1)

How do __init__ and bar get that self parameter?

Unbound methods

Well, bar is just a plain old function.

3

Never call readlines() on a file

Calling readlines() makes your code slower, less explicit, less concise, for absolutely no benefit.

There are hundreds of questions on places like StackOverflow about the readlines method, and in every case, the answer is the same.

6

You don't actually have to be a dummy to not get list comprehensions. Only a few languages (after Python, the next most popular is probably Haskell) support them. And they're only "easy" once you learn to think a different way.

You do eventually want to learn to think that way.

The first time you try to write a network client or server directly on top of sockets, you do something like this:

for filename in filenames: with open(filename, 'rb') as f: sock.sendall(f.read())

And then, on the other side:

for i in count(0): msg = sock.recv(1<<32) if not msg: break with open('

9

(Someone pointed out to me that Ned Batchelder has a similar post called Keep data out of your variable names.

7

Novices to Python often come up with code that tries to build and evaluate strings, like this:

for name in names: exec('{} = {}'.format(name, 0))

… or this:

exec('def func(x): return {} * x**2 + {} * x + {}'.format(a, b, c)) return func

99% of the time, this is a problem you shouldn't be solving

If you look at the major changes in Python 3, other than the Unicode stuff, most of them are about replacing list-based code with iterator-based code.

2

Let's say you've got the list [-10, 3, -2, 14, 5], and you want the list filtered to just the non-negative values.

2

Why install a third-party Python 2.7?

Python.org, Homebrew, scipy.org, 69105 different blog posts explaining how to install numpy/scipy/etc. on a Mac, etc. all tell you to install a different Python alongside Apple's.

tl;dr

Install Xcode from the App Store, then install the Command Line Tools from Xcode (Preferences | Downloads | Components). Install Python 3.3.0 (or the latest non-beta version, if you're in the future) by running the "Mac OS X 64-bit/32-bit Installer".

tl;dr

Install Xcode from the App Store, then install the Command Line Tools from Xcode (Preferences | Downloads | Components).

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