Week 14

More About Applications and More Review

CSV file exercise (from last week)

Write a function csv_column that takes the name of a CSV file and a column number (one-based indexing) and prints the elements in the column.

Recall: list(csv.reader(csv_file_object)) returns table representing the contents of the CSV file.

CSV Column Application Exercise

Extend the solution from last week to create an application that prompts for the input file and column number, and then invokes this function.

Repeat the column number prompt if a positive integer is not entered. Recall the string method isdigit().

import csv

def csv_column(csv_file_name, column_number):
    """Print the strings in the indicated column (one-based indexing) in
    the named CSV file, excluding empty strings.
    """
    # Hint: list(csv.reader(<file object>))
    csv_file = file(csv_file_name, 'r')
    table = list(csv.reader(csv_file))
    csv_file.close()
    column_number = column_number - 1
    for index in range(len(table)):
        row = table[index]
        value = row[column_number]
        if value.strip() != '':
            print value

CSV Column Application Solution

import csv

def csv_column(csv_file_name, column_number):
    """Print the strings in the indicated column (one-based indexing) in
    the named CSV file, excluding empty strings.
    """
    # Hint: list(csv.reader(<file object>))
    csv_file = file(csv_file_name, 'r')
    table = list(csv.reader(csv_file))
    csv_file.close()
    column_number = column_number - 1
    for index in range(len(table)):
        row = table[index]
        value = row[column_number]
        if value.strip() != '':
            print value

def main():
    """Prompt for csv file name and column number, and then call
    the csv_column function. Repeat the column number prompt if a positive
    integer is not entered.
    """
    file_name = raw_input('CSV file name: ')
    while True:
        column = raw_input('Column number: ')
        if column.isdigit():
            column_number = int(column)
            if column_number > 0:
                break
    csv_column(file_name, column_number)

main()

OS shell command line arguments

• Operating system command text is broken into whitespace-separated command-line arguments by the shell and passed to the program
  • the OS shell is really an interpreter for a special language, in which running a user program is analogous to a function call
  • the syntax is quite different from that of most general purpose languages, such as Python
• If an argument contains spaces or other characters with special meaning to the shell, the argument must be quoted with single or double quotes

OS shell command arguments and Python

• In Python, command-line arguments are available as a list of string in the variable argv of module sys

  • the program name is in sys.argv[0] and the arguments that followed the program name are in sys.argv[1:]
  • the sys module must be imported first

• OS shell demo:

  > type args.py
  import sys
  print sys.argv
  
  > python args.py arg1 "argument two" 3
  ['args.py', 'arg1', 'argument two', '3']
  

• FYI

  • sys stands for system and the module also contains other system specific information
  • argv stands for argument vector ("vector" is an old term meaning about the same thing as "list" or "array")

CSV Column Application with command line arguments

import csv, sys

def csv_column(csv_file_name, column_number):
    """Print the strings in the indicated column (one-based indexing) in
    the named CSV file, excluding empty strings.
    """
    # Hint: list(csv.reader(<file object>))
    csv_file = file(csv_file_name, 'r')
    table = list(csv.reader(csv_file))
    csv_file.close()
    column_number = column_number - 1
    for index in range(len(table)):
        row = table[index]
        value = row[column_number]
        if value.strip() != '':
            print value

def main():
    """Usage: python csv_column3.py <csv file name> <column number>
    """
    if len(sys.argv) == 3:
        csv_file_name = sys.argv[1]
        column = sys.argv[2]
        if column.isdigit():
            column_number = int(column)
            if column_number > 0:
                csv_column(csv_file_name, column_number)
                return
        print "Error: bad column number"
    else:
        print main.__doc__ # print the function documentation string

main()

Output file argument

We add an output file name argument to our application, so output goes to a file rather than being printed.

import csv, sys

def csv_column(csv_file_name, column_number, output_file_name):
    """Write the strings in the indicated column (one-based indexing) in
    the named CSV file, one per line, excluding empty strings. Write to
    the named output file.
    """
    # Hint: list(csv.reader(<file object>))
    csv_file = file(csv_file_name, 'r')
    table = list(csv.reader(csv_file))
    csv_file.close()

    output_file = file(output_file_name, 'w')
    column_number = column_number - 1
    for index in range(len(table)):
        row = table[index]
        value = row[column_number]
        if value.strip() != '':
            output_file.write(value + '\n')
    output_file.close()

def main():
    """Usage: python csv_column4.py <csv file name> <column number> <output file>
    """
    if len(sys.argv) == 4:
        csv_file_name = sys.argv[1]
        column = sys.argv[2]
        output_file_name = sys.argv[3]
        if column.isdigit():
            column_number = int(column)
            if column_number > 0:
                csv_column(csv_file_name, column_number, output_file_name)
                return
        print "Error: bad column number"
    else:
        print main.__doc__ # print the function documentation string

main()

Which of the following is not an expression (in Python)?

1. a[3]
2. a[3, 4]
3. a(3,4)
4. a[3:4]
5. a(3)
6. none of the above: they are all expressions

Answer: B

Which of the following is not an expression in Python?

1. [3]
2. [3, 4]
3. [3:4]
4. (3)
5. none of the above: they are all expressions

Answer: C

How much do you think Alice helped you learn programming?

1. A lot
2. A bit
3. Very little
4. Not at all

Continue using Alice?

Do you think it would it be more helpful for most students to not use Alice?

The course would then probably start with a bit more Reeborg programming, a little slower pace with the current Python material, and perhaps some Python graphics programming at the end of the course.

1. Yes
2. Maybe so
3. Probably not
4. No

FYI: Standard output

• sys.stdout is a file object to which printed output is sent
• sys.stdout.write(string + 'n') is the same as print string

Standard output example

• We add an optional command-line argument to our csv column application to indicate the file to which output is to be sent. It defaults to standard output (same as printing)

import csv, sys

def csv_column(csv_file_name, column_number, output_file_name):
    """Write the strings in the indicated column (one-based indexing) in
    the named CSV file, one per line, excluding empty strings. Write to
    the named output file.
    """
    # Hint: list(csv.reader(<file object>))
    csv_file = file(csv_file_name, 'r')
    table = list(csv.reader(csv_file))
    csv_file.close()

    output_file = file(output_file_name, 'w')
    column_number = column_number - 1
    for index in range(len(table)):
        row = table[index]
        value = row[column_number]
        if value.strip() != '':
            output_file.write(value + '\n')
    output_file.close()

def main():
    """Usage: python csv_column4.py <csv file name> <column number> <output file>
    """
    if len(sys.argv) == 4:
        csv_file_name = sys.argv[1]
        column = sys.argv[2]
        output_file_name = sys.argv[3]
        if column.isdigit():
            column_number = int(column)
            if column_number > 0:
                csv_column(csv_file_name, column_number, output_file_name)
                return
        print "Error: bad column number"
    else:
        print main.__doc__ # print the function documentation string

main()

FYI Raising and handling exceptions

• Exceptions may be raised (generated) by the system, or by the programmer using a raise statement
• If the programmer does not handle an exceptions, the system typically prints an error message and then terminates the program
• With a try statement, the programmer may handle exceptions so that a different message may be printed and the program may continue somehow instead of terminating