Last Week

• Lists

• List methods

• Nested Lists

• Looping through lists using for loops

This Week and Maybe NextWhile loops

Special characters in strings

Files – What are they?– Opening and closing files– Reading and writing to files– Comma Separated Files

While Loops

Sometimes we need to loop until a condition is met.

For example:

– Ask user for a password twice– If the passwords don’t match, ask again

until they match or the user quits

While Loop Example

English example:

Ask for password.Ask for password again.While the passwords don’t match:

Ask again.

While Loop Example

Python example:

password1 = input(‘Enter password: ’)Ask for password again.While the passwords don’t match:

Ask again.

While Loop Example

Python example:

password1 = input(‘Enter password: ’)password2 = input(‘Re-enter password: ’)While the passwords don’t match:

Ask again.

While Loop Example

Python example:

password1 = input(‘Enter password: ’)password2 = input(‘Re-enter password: ’)while password1 != password2:

Ask again.

While Loop Example

Python example:

password1 = input(‘Enter password: ’)password2 = input(‘Re-enter password: ’)while password1 != password2:

password2 = input(‘Re-enter password:’)

Invisible CharactersHow do we indicate a tab, newline or ‘ or “ inside a string?

\t tab \” double quote\n new line\’ single quote \\backslash

These are called escape sequences.print(“This string has a \n newline and a \t tab.”

This string has a

newline and a tab.

Files• Files – what are they?

– Can think of as a collection of stored information

– Computer thinks as a sequence of bits possible arranged in characters

• Files have names: – my_python.py, homework.txt, etc.

• What can we do with a file? – Read from a file and write to a file

Opening FilesHow do we open a file in Python?

import doctest

myfile = open(‘story.txt’, ‘r’)

•Open is the Python function

•Story.txt is the name of the file to be opened•myfile is a variable that is assigned the file object (also called stream or reader).•‘r’ is a string indicating what we will do with the file (read, write, append)

Using FilesAfter we are finished with a file we must close it:

myfile.close()

When we write to a file, we have two choices:Write: myfile = open(‘filename’, ‘w’)Append: myfile = open(‘filename’, ‘a’)

•write replaces the file filename•append appends to the file filename

Reading FilesThere are many ways to read from a file.

1.Using a for loop:myfile = open(‘filename’, ‘r’)

for line in myfile:

<do something with the line>

2.Read the whole file at once into a list of strings:

myfile = open(‘filename’, ‘r’)

list_of_lines = myfile.readlines()

Reading Files

3.Read the entire file into a string:myfile = open(‘filename’, ‘r’)

s = myfile.read()

4.Read a specific number of characters:myfile = open(‘filename’, ‘r’)

s = myfile.read(10) reads 10 characterss = myfile.read(10) reads then next 10 characters

Reading Files

5.Read one line at a time:myfile = open(‘filename’, ‘r’)

line = myfile.readline() reads a lineline = myfile.readline() reads the next line

End Of File (EOF)How do we know when we reach the end of the file?

This method automatically recognizes EOF.for line in myfile:

<do something with the line>

Here we have to check for the end of file:line = myfile.readline() reads the next lineand s = myfile.read(10) reads 10 characters

The EOF character is the empty string “”.

Writing to a fileFirst open the file for writing or appending:

myfile = open(‘story.txt’, ‘w’) start the storymyfile = open(‘story.txt’, ‘a’) continue the story

Then write to the file:myfile.write(‘Once upon a time…’)

myfile.write(‘The end.’)

Then close the file:

myfile.close()

Reading from a CSV file

Often we have comma-separated values data files:

First Name, Last Name, Utorid

Anna, Bretscher, bretsche

Joe, Johnson, johnsonj

Sally, Jordan, jordansa

Why do we like CSV files?–Spreadsheet applications like excel understand it–Many other programming languages also understand them–Easy to generate ourselves

Reading from a CSV file

How do we read comma-separated values data files:

import io

import csv

csv_file = open(‘csv_filename.csv’, ‘r’)

reader = csv.reader(csv_file)

…

for line in reader:

# read like an ordinary file

# but line is a list