Python - write variable created in a loop into output file

Question

I have a function which takes a nested list that I input and writes it into the console in the format that I am after.

def print_table(table):
    longest_cols = [(max(
        [len(str(row[i])) for row in table]) + 2) 
        for i in range(len(table[0]))]
    row_format = "".join(["{:>" + str(longest_col) + "}" 
        for longest_col in longest_cols])
    for row in table:
        print(row_format.format(*row))

How would I modify the function so that it writes the output into an output file?

I attempted this by saying

x = print_table(table)

and then

f.write(x)
f.close()

But all this did is write none into the output file

Any help in this is greatly appreciated. Thank you!

This may help... just direct output to a file in append mode: directing output to a text file — Fubar
– Fubar, Commented Apr 8, 2020 at 17:44
You need a return row_format line at the end of a function. if you don't return anything, the result of a function call is None — tdelaney
– tdelaney, Commented Apr 8, 2020 at 17:48

Joshua Varghese · Accepted Answer · 2020-04-08 18:09:13Z

3

When you define a function and call it, you have to use return to assign it to something.
But if its row_format.format(*row) you want to store, open it in the function:

def print_table(table,f):
    longest_cols = [ (max([len(str(row[i])) for row in table]) + 2) for i in range(len(table[0]))]
    row_format = "".join(["{:>" + str(longest_col) + "}" for longest_col in longest_cols])
    for row in table:
        f.write(row_format.format(*row))
    f.close()

Now just call it:

print_table(table,f)

Lets say, you want to add it line by file, then use:

for row in table:
    f.seek(0)
    f.write("\n") #not possible if file opened as byte
    f.write(row_format.format(*row))

Now if you want it your way, try:

def print_table(table):
    longest_cols = [(max(
        [len(str(row[i])) for row in table]) + 2) 
        for i in range(len(table[0]))]
    row_format = "".join(["{:>" + str(longest_col) + "}" 
        for longest_col in longest_cols])
    return '\n'.join(row_format.format(*row) for row in table)

Now call it:

x = print_table(table)
f.write(x)
f.close()

edited Apr 8, 2020 at 18:09

answered Apr 8, 2020 at 17:43

Joshua Varghese

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6 Comments

tdelaney Over a year ago

Where did f come from?

Joshua Varghese Over a year ago

f crom from? what did you mean?

gnodab Over a year ago

The line with open(....) as f:. Although I don't think you need the f.close() at the end.

Joshua Varghese Over a year ago

Ok. then what would f be in the question?

Joshua Varghese Over a year ago

Does then the file get saved? isint f.close(0 required for the file to save?

|

tdelaney · Accepted Answer · 2020-04-08 17:59:31Z

There are lots of ways to solve this, depending on what responsibilities you want your function to have. You could have the function format the table but leave the output to the caller (this may be more generally useful if the caller wants the formatted table to go to different places)

def print_table(table):
    longest_cols = [(max(
        [len(str(row[i])) for row in table]) + 2) 
        for i in range(len(table[0]))]
    for longest_col in longest_cols:
        yield "".join(["{:>" + str(longest_col) + "}" 

with open("foo.txt", "w") as f:
    f.writelines(row + "\n" for row in print_table(table))

Or you could give output responsibility to the function and pass it the output stream you want

import sys

def print_table(table, file=sys.stdout):
    longest_cols = [(max(
        [len(str(row[i])) for row in table]) + 2) 
        for i in range(len(table[0]))]
    row_format = "".join(["{:>" + str(longest_col) + "}" 
        for longest_col in longest_cols])
    for row in table:
        print(row_format.format(*row), file=file)

with open("foo.txt", "w") as f:
    print_table(table, f)

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