Python crib sheet #1

Some notes I've been making as I learn Python.

Packages

The __init__.py file is "automatically executed by Python the first time a package or subpackage is loaded. This permits whatever package initialization you may desire. Python requires that a directory contain an __init__.py file before it can be recognized as a package. This prevents directories containing miscellaneous Python code from being accidentally imported as if they defined a package.

"The second point is probably the more important. For many packages, you won’t need to put anything in the package’s __init__.py file—just make sure an empty __init__.py file is present." [1]

"Note that there’s no recursive importing of names with a from ... import * statement." [1]

"A two-level hierarchy should be able to effectively handle all but a few of the rest. As written in the Zen of Python, by Tim Peters, “Flat is better than nested.”" [1]


Underscores

In general, single underscores are a convention, double has a meaning (name mangling).
"Names, in a class, with a leading underscore are simply to indicate to other programmers that the attribute or method is intended to be private. However, nothing special is done with the name itself.

"Any identifier of the form __spam (at least two leading underscores, at most one trailing underscore) is textually replaced with _classname__spam, where classname is the current class name with leading underscore(s) stripped...  Name mangling is intended to give classes an easy way to define “private” instance variables and methods, without having to worry about instance variables defined by derived classes, or mucking with instance variables by code outside the class. Note that the mangling rules are designed mostly to avoid accidents; it still is possible for a determined soul to access or modify a variable that is considered private." (StackOverflow)

A special example is the __getitem__ special method attribute. "A solution is to use the __getitem__ special method attribute, which you can define in any user-defined class, to enable instances of that class to respond to list access syntax and semantics." [1]

Numbers

"Python offers four kinds of numbers: integers, floats, complex numbers, and Booleans.

"An integer constant is written as an integer—0, –11, +33, 123456—and has unlimited range, restricted only by the resources of your machine.

"A float can be written with a decimal point or using scientific notation: 3.14, –2E-8, 2.718281828. The precision of these values is governed by the underlying machine but is typically equal to double (64-bit) types in C.

"Complex numbers are probably of limited interest... Booleans are either True or False and behave identically to 1 and 0 except for their string
representations." [1, p40]


Pythonic manipulation of lists

For flatten, see this StackOverflow answer.

flat_list = [item for sublist in l for item in sublist]

which means:

for sublist in l:
    for item in sublist:
        flat_list.append(item)

where l is the structure we want to flatten.

Note that the += you'd find in Scala is not the same in Python. From StackOverflow.

+ is closer in meaning to extend than to append... the methods work in-place: extend is actually like += - in fact, it has exactly the same behavior as += except that it can accept any iterable, while += can only take another list.

Given a value x that we want i times, a repeating list can be created with [x] * i.


Pass

"The pass statement does nothing particular but can act as a placeholder" (StackOverflow). This makes the code compile and appears to act like a placeholder like ??? in Scala.

[1] The Quick Python Book, Second Edition