This presentation is how to do simple file reads and writes in python3

1. Computer Science 151
An introduction to the art of computing
File Input and Ouptut
Rudy Martinez

2. CS151 Spring 2019
Opening Files in Python
● Open() function
○ Returns a file object
○ myFile = open(‘filename’, ‘mode’)
■ Mode:
● ‘r’ - Read only
● ‘w’ - write, allows you to write and edit files
○ This will overwrite an existing file! Be careful.
● ‘a’ - append will add to the end of the file.
myfile=open(‘Moby_Dick_by_Mellville.txt’, ‘r’)

3. CS151 Spring 2019
Using a file object
myfile=open(‘Moby_Dick_by_Mellville.txt’, ‘r’)
● To read a single line.
○ myfile.readline()
■ This returns a string
● To read all lines into a list.
○ textList = myfile.readlines()
■ This returns a list

4. CS151 Spring 2019
How to read all lines
● textList = myfile.readlines()
● How do we go through a list?
○ For loop!
# Open file
myfile=open('/home/rudym/Downloads/Moby_Dick_by_Melville.txt', 'r')
# Load each line in file to list textList
textList = myfile.readlines()
# For loop to read and print each line
for i in range(0, len(textList), 1):
print(textList[i])

5. CS151 Spring 2019
Writing Files
● Writing files is similar to reading files.
● The main misstep is using ‘w’ instead of ‘a’ when you have a file you want to
add to.
○ ‘w’ will write over any file
○ ‘a’ will append to the end of the file preserving previous entries
outfile = open(‘Filename’, ‘w’)

6. CS151 Spring 2019
Writing files
● Also like read, there are two main functions for putting text into a file.
○ write(‘string’) - will write a passed string into the file and does not append a carriage return
○ writelines( string, list, or dictionary ) - takes anything that can be iterated
■ Iterables are strings, list and dictionaries
outfile = open(‘test.txt’, ‘w’)
for i in range(0, 10, 1):
outfile.write(str(i))
Output: 0123456789
outfile = open(‘test.txt’, ‘w’)
myString = ‘Hello Cruel World’
outfile.writelines(myString)
Output: Hello Cruel World

7. CS151 Spring 2019
Iterables
● An iterable is any data structure that has an iterator
● What is an iterator?
○ An iterator is a pointer that walks any data structure.
Index 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16
myString H e l l o C r u e l W o r l d
[H]

8. CS151 Spring 2019
Iterables
● An iterable is any data structure that has an iterator
● What is an iterator?
○ An iterator is a pointer that walks any data structure.
Index 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16
myString H e l l o C r u e l W o r l d
[He]

9. CS151 Spring 2019
Iterables
● An iterable is any data structure that has an iterator
● What is an iterator?
○ An iterator is a pointer that walks any data structure.
Index 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16
myString H e l l o C r u e l W o r l d
[Hel]

10. CS151 Spring 2019
Iterables
● An iterable is any data structure that has an iterator
● What is an iterator?
○ An iterator is a pointer that walks any data structure.
Index 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16
myString H e l l o C r u e l W o r l d
[Hell]

11. CS151 Spring 2019
Iterables
● An iterable is any data structure that has an iterator
● What is an iterator?
○ An iterator is a pointer that walks any data structure.
Index 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16
myString H e l l o C r u e l W o r l d
[Hello]

12. CS151 Spring 2019
Iterables
● An iterable is any data structure that has an iterator
● What is an iterator?
○ An iterator is a pointer that walks any data structure.
Index 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16
myString H e l l o C r u e l W o r l d
[Hello ]

13. CS151 Spring 2019
Iterables
● An iterable is any data structure that has an iterator
● What is an iterator?
○ An iterator is a pointer that walks any data structure.
Index 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16
myString H e l l o C r u e l W o r l d
[Hello C]

14. CS151 Spring 2019
Iterables
● An iterable is any data structure that has an iterator
● What is an iterator?
○ An iterator is a pointer that walks any data structure.
Index 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16
myString H e l l o C r u e l W o r l d
[Hello Cr]

15. CS151 Spring 2019
Iterables
● An iterable is any data structure that has an iterator
● What is an iterator?
○ An iterator is a pointer that walks any data structure.
Index 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16
myString H e l l o C r u e l W o r l d
[Hello Cru]

16. CS151 Spring 2019
Iterables
● An iterable is any data structure that has an iterator
● What is an iterator?
○ An iterator is a pointer that walks any data structure.
Index 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16
myString H e l l o C r u e l W o r l d
[Hello Crue]

17. CS151 Spring 2019
Iterables
● An iterable is any data structure that has an iterator
● What is an iterator?
○ An iterator is a pointer that walks any data structure.
Index 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16
myString H e l l o C r u e l W o r l d
[Hello Cruel]

18. CS151 Spring 2019
Iterables
● An iterable is any data structure that has an iterator
● What is an iterator?
○ An iterator is a pointer that walks any data structure.
Index 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16
myString H e l l o C r u e l W o r l d
[Hello Cruel ]

19. CS151 Spring 2019
Iterables
● An iterable is any data structure that has an iterator
● What is an iterator?
○ An iterator is a pointer that walks any data structure.
Index 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16
myString H e l l o C r u e l W o r l d
[Hello Cruel W]

20. CS151 Spring 2019
Iterables
● An iterable is any data structure that has an iterator
● What is an iterator?
○ An iterator is a pointer that walks any data structure.
Index 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16
myString H e l l o C r u e l W o r l d
[Hello Cruel Wo]

21. CS151 Spring 2019
Iterables
● An iterable is any data structure that has an iterator
● What is an iterator?
○ An iterator is a pointer that walks any data structure.
Index 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16
myString H e l l o C r u e l W o r l d
[Hello Cruel Wor]

22. CS151 Spring 2019
Iterables
● An iterable is any data structure that has an iterator
● What is an iterator?
○ An iterator is a pointer that walks any data structure.
Index 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16
myString H e l l o C r u e l W o r l d
[Hello Cruel Worl]

23. CS151 Spring 2019
Iterables
● An iterable is any data structure that has an iterator
● What is an iterator?
○ An iterator is a pointer that walks any data structure.
Index 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16
myString H e l l o C r u e l W o r l d
[Hello Cruel World]

24. CS151 Spring 2019
For loops revisited!
Old For loop
for i in range(0, len(myString), 1):
print(myString[i])
Quick Loop
for i in myString:
print(i]
The two for loops below do the exact same thing, but one is short hand!
Why didn’t you teach us this first?!?*?

25. CS151 Spring 2019
To Sum it up
● Open returns a file object for reading and writing.
● Reading:
○ readline() - Single line of text
○ readlines() - All lines into list
● Writing:
○ writeline() - Single string to file
○ writelines() - An iterable to the file

26. CS151 Spring 2019
Friday
● 2/15 Homework 2 part A assigned
● 2/20 Homework 2 part A DUE! (5 days)
● 2/22 Homework 2 part B assigned
● 2/27 Homework 2 part B DUE! (5 days)
● 3/1 Homework 2 part C assigned
● 3/4 Exam Review!!!
● 3/6 Midterm Exam
● 3/29 Homework 2 part C DUE! (nearly a month)