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Python Basics

Iterators

Iterator vs. Iterable

This is a commonly asked question. In general, an Iterable object can be anything, as long as it implements the standard __iter()__ method. Python lists, tuples, maps, and of course, iterators, all implement this method. You can think of Iterable as an interface or protocol.

The Iterator on the other hand, is a one specific implementation of the Iterable protocol, which tries to be memory-efficient, thus its values can be accessed one at a time. Iterators implement an additional method called __next()__, which gets called upon every pass, and returns one value at a time. Iterators are all over the placein Python, and pretty much every collection type (e.g. map, list, etc) can be turned into and Iterator, by wrapping the collection with the iter() function. Additionaly, map, filter, and also, functions using yield, all return Iterator instances.

Accessing the Index of an Iterable Row

If you are looping over an iterator or a list, often you'd like to get the index of the particular row you're on. This is where the enumerate function comes handy:

# assuming that items is an iterable object
for idx, item in enumerate(items):
    print(idx, item)

enumerate1

Return an enumerate object. iterable must be a sequence, an iterator, or some other object which supports iteration. The __next__() method of the iterator returned by enumerate() returns a tuple containing a count (from start which defaults to 0) and the values obtained from iterating over iterable.

Dates and Date Formatting

Turn a UNIX timestamp into a date object / date string 2

from datetime import datetime
timestamp = 1545730073
dt_object = datetime.fromtimestamp(timestamp)

str_date = str(dt_object) # '2018-12-25 09:27:53'

Setup and Installing Dependencies

requirements.txt

Requirements files can invoke other requirements files

In a very simple form of inheritance requirements.txt can get the dependencies of other requiements.txt files, and add additional ones on top. How is this useful? Let's say that you keep your common dependencies in requirements.txt and have additional tooling and dependencies in a separate file, let's call it requirements-dev.txt. When you do your local setup instead of doing the usual drill:

pip install -r requirements.txt

you will do the following:

pip install -r requirements-dev.txt

All you need to do, in order for you to get all the original dependencies listed in requirements.txt, is to add one line to requirements-dev.txt:

-r requirements.txt

# Add your other developer and tooling dependencies underneath