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.

Python defines context using the with statement. The following program is a small example that parses a log file to create a useful CSV summary of that log. Since there are two open files, this will use the nested with contexts. The example uses a complex regular expression, format_1_pat. We'll define this shortly.

We might see something similar to the following in an application program:

from pathlib import Path
import gzip 
import csv 

source_path = Path.cwd()/"data"/"compressed_data.gz"
target_path = Path.cwd()/"data"/"subset.csv"

with target_path.open('w', newline='') as target: 
    wtr= csv.writer( target ) 
    with gzip.open(source_path...