利用DBI模块将Perl代码与MySQLdb转换为python

Question

I have little python experience, and I have zero perl knowledge, but I need to translate a perl code that connects to a MySQL server into python.

In the perl subroutine, after the standard connectivity actions, the code that appears is:

my @tmp = @{$dbh->selectall_arrayref( $sql )};

my @types;
foreach my $t (@tmp) {
push @types, @$t[0];
}

return @types;

where $sql is the MySQL selection query. What I would do in a python function is:

cursor = conn.cursor()
cursor.execute( sql )
tmp = cursor.fetchall()
types = list( len( tmp ) )
for item in tmp :
    types.append( item[0] )

return types

My question is what does @$t[0] contain and is that equivalent to the item[0] in tmp?

Thanks!

Answer 1

You should be able to run the code to discover equivalency, but yes those two things are equivalent.

The thing that causes the syntax to be odd in the perl example is that perl had a very strict separation of scalars, arrays, and hashes in its early days and arrays and hashes could only have scalars as their stored values. This was quickly improved by adding references into perl which are scalars that point to arrays and hashes, but the syntax was already set with that separation so additional operations had to be added to the language.

As a result, the first line is taking a reference to an array of array references and dereferencing the outer reference so that it is now an array of array references. It then loops over all those references inside it, dereferences them and takes the first element in the array. Because @$t[0] looks odd usually you would see it written as $t->[0] which doesn't require you to know that the array dereference operator ( @ ) binds tighter than the element access operator ( [0] ).