Question

I’m having trouble understanding why an inner loop in my method isn’t producing the desired behavior I’m expecting and I’m hoping someone can help me understand the problem.

My method takes a series of arguments (*args) and if the argument is an integer I want to add dollar signs around the integer (eg. $5$).

def t_row(*args):
    columns = 5
    if len(args) == columns:
        count = 0
        for value in args: 
            if type(value) is int:
                value = ''.join(('$', str(value), '$'))
            count += 1
            if count < len(args):
                penult_args = args[:-1]
                line_prefix = [''.join((str(value), " & ")) for value in penult_args]
            elif count == len(args):
                line_suffix = [''.join((str(value), " \\\\", "\n"))]
        count += 1
        line_list = line_prefix + line_suffix 
        line = ''.join(item for item in line_list)
        return(line)

The above code is used like this:

>>> s = q.t_row('data1', 'data2', 3, 'data4', 5)  
>>> print s  
data1 & data2 & 3 & data4 & $5$ \\  

Why don’t I get dollar signs around the integer 3? How can I fix my code to correct this problem?

Answer 1

Because on this line:

            line_prefix = [''.join((str(value), " & ")) for value in penult_args]

you pull the values out of the original list (minus the last item), while on this line:

            value = ''.join(('$', str(value), '$'))

You added $ but never stored the value back into the list.

The 5 only gets $ because it’s the last item, so you reference it directly in:

            line_suffix = [''.join((str(value), " \\\\", "\n"))]

A better way to do all this is:

def t_row(self, *args):
    if len(args) == self.columns:
        result = []
        for value in args:
            if isinstance(value, int):
                result.append('$%d$' % value)
            else:
                result.append(value)
        return ' $ '.join(result) + r' \\'

As a one-liner, it would be

t_row = lambda self, *args: (' $ '.join('$%d$' % 
          value if isinstance(value, int) else value for value in args) + r' \\' 
              if len(args) == self.columns else None)

but that’s not actually a good idea.