cat, Python, and Perl

2011-03-15

I came across a tutorial that showes how to implement a simple cat in Perl. I've modified their example for the sake of simplicity:

sub cat {
    foreach my $filename (@_) {
        open FILE, $filename;
        while (my $line = <FILE>) {
            print $line;
        }
    }
}
cat @ARGV;

Running this script (e.g. perl cat.pl file) will display those files as if you ran cat file.

I decided to see how the Python example would look like:

import sys
def cat(files):
    for filename in files:
        with open(filename) as FILE:
            for line in FILE:
                print(line, end="")
cat(sys.argv[1:])

Running this script (e.g. python3 cat.py file1 file2) will give exactly the same result as above.

notes

The Perl keyword, my, indicates that the variable it refers to is local. Without that, it's taken to be global. As for Python, this is implicit, and depends on where the variable is placed (scope).
Perl has special ways of identifying data types. In our example, $ is used to identify a variable that has a single value, and is known as a scalar in Perl talk. This can be tedious of course (so much typing!). In Python, the data fed into the variable is the only thing that determines what type of the variable it is. I see this as noise from Perl. It results in some badness, as in you can have two variables names be the same, but given different types (e.g. my $var; my @var;)
The upper case file handler, FILE, is a matter of convention for Perl, and can be named 'anything'. I used it in the Python sample only for the sake of clarity.
The <> operator is special syntax that means a file is being manipulated. Python has no such.
The @_ is an argument list (@ARGV) from the function call, cat. This is one other thing that is implicit about Perl, where you have to learn extra syntax (and concepts), where a simple argument list should have been provided during the function declaration, as in Python's cat(files).
Note that I didn't need to import anything to get Perl to work with command line arguments. With Python, I need to explicitly do so, and that's via the sys module, which is part of its standard library.
The Python with statement is meant to make our lives easier, but also adds syntax to the language. What it does is close a file for us so we don't have to do it.
The Python print() function adds a newline by default, and that would results in ugly output from our code, that's why we used the end="".
The [1:] from the last statement in the Python example means that we are slicing the list, removing the first element (element 0), and keeping everything else. We do this because the current running script is considered by Python as element 0, while in Perl, the 1st element is actually the first thing that appears on the command line after the script name. One would say that Perl does this more elegantly.

notes

further reading