Managing contexts and the with statement

Contexts and context managers are used in several places in Python. We'll look at a few examples to establish the basic terminology.

A context is defined by the with statement. The following program is a small example that parses a logfile to create a useful CSV summary of that log. Since there are two open files, we expect to see nested with contexts. The example uses a complex regular expression, format_1_pat. We'll define this shortly.

We might see something like the following in an application program:

import gzip
import csv
with open("subset.csv", "w") as target:
    wtr= csv.writer( target )
    with gzip.open(path) as source:
        line_iter= (b.decode() for b in source)
        match_iter = (format_1_pat.match( line ) for line in line_iter)
        wtr.writerows( (m.groups() for m in match_iter if m is not None) )

Two contexts with two context managers were emphasized in this example.

The outermost context starts with with open("subset.csv",...