Python tricks

2016-10-31

To start reading in lines from a certain line number in Python you can use enumerate and its start parameter (via)[https://www.youtube.com/watch?v=EnSu9hHGq5o]:

with open('filename.txt') as f:
    for linenum, line in enumerate(f, start=10):
        # ...

Use a generator to simplify filtering of lines when reading a file:

def interesting_lines(f):
    for line in f:
        line = line.strip()
        if line.startswith('#'):
            continue
        if not line:
            continue
        yield line

with open ('filename.txt') as f:
    for line in interesting_lines(f):
        do_something(line)

The interesting_lines() function can be tested using a list of strings, it doesn't depend on reading a file.

How to break out of two loops? Make the double loop single:

def range_2d(width, height):
    for y in range(height):
        for x in range(width):
            yield x, y

Make your objects iterable:

class ToDoList:
    def __init__(self):
        self.tasks = []

    def __iter__(self):
        for task in self.tasks:
            if not task.done:
                yield task

    def all(self):
        return iter(self.tasks)

    def done(self):
        return (t for t in self.tasks if t.done)


todo = ToDoList()
...
for task in todo:
    # ...