Tuples & Files

CSCI 1012

Announcements

  • Homework 9 (tuples) is due Sunday, 6 Apr 11:55 PM
  • Common reasons for losing points on quizzes:
    • Not tracing
    • (That’s the reason.)

In-Class Exercises

  • File names:
    • ex1.py
    • ex2.py
    • ex3.py
  • Turn in on the submit server before the end of lecture for credit
  • Turn in one .zip with all the .py files
    • Unlimited attempts during lecture
  • We’ll provide solutions – pay attention!

Common Errors

  • Incorrect file names:
    • check_even.py vs. checkEven.py
  • Incorrect function name
    • def check_even(x): vs. def check_evens(x):
  • Spacing errors
    • 1 2 3 vs. 1 2 3

Read your autograder output!

Today

  • Tuples
  • Tuple assignment
  • Tuples vs. lists
  • Files

Warmup

  • Lists are mutable, ordered collections
  • Lists are passed as references
    • Modifying a list in a function can modify the list outside of the function

Exercise 1

Write a function ex1 that takes a single list argument, and returns a new list, consisting of the same contents as the original list, but with an integer 9 appended to it. Do not modify the input list.

  • ex1([2, 3]) returns [2, 3, 9]
L = [4, 6, 7]
ex1(L)
print(L)

prints [4, 6, 7] (original list not modified)

Submit as ex1.py.

Tuples

  • Superficially similar to lists
  • Tuples use parentheses ( and )
char_tuple = ("A", "C", "D", "F")
  • Tuples can be indexed just like lists
print(char_tuple[2])
D
  • Difference between tuples and lists: tuples are immutable

Tuples Are Immutable

List:

L = ["A", "B", "C"]
L[1] = "E"
print(L)
['A', 'E', 'C']

Tuple:

K = ("A", "B", "C")
K[1] = "E"
print(K)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: 'tuple' object does not support item assignment

Syntax 😎

  • Lists are defined with square brackets [ and ]
x = [2, 3, 4]
  • Tuples are defined with parentheses ( and )
y = (3, 4, 5)
  • Dicts are defined with curly braces { and }
z = {"A": 1, 2: 3}

Syntax 😣

  • Lists, Tuples, Dicts, and Strings are all indexed with square brackets
w = "confounded!" # string

x = [2, 3, 4] # list

y = (3, 4, 5) # tuple

z = {"A": 1, 2: 3} # dict

print(w[2], x[0], y[1], z["A"])
n 2 4 1

Syntax 😖

  • Parentheses are used in:
    • Expressions
    • Function definitions
    • Function calls
def f(x, y):
  z = str(x) + ( str(y) * 2 )
  return z

w = f(3, 5)
print(w)
355

Syntax 😩

…parentheses for tuple definitions are often optional

x = 1, 2, 3
print(x)
print(type(x))
(1, 2, 3)
<class 'tuple'>

With parentheses:

x = (1, 2, 3)
print(x)
print(type(x))
(1, 2, 3)
<class 'tuple'>

Syntax 😤

  • Square brackets after a name indicates an index or slice, not a list:
    • x[2] or y["A"]
  • Parentheses after a name indicate a call:
    • f(4, 5) or print("hello", "world")
  • Curly braces are always a dictionary definition:
    • z = {1: 2}
    • Other uses: Python we don’t teach in CSCI 1012

Syntax 😤

  • Function definitions always have the def keyword

  • Parentheses not after a name and with a comma indicate a tuple:

    • (2, 3) or ("yes",)

  • Parentheses not after a name with no comma indicate an expression:

    • (5) or ("otis")

Concatenation

Recall strings:

s = "hello"
s = s + " world"
print(s)
hello world

Tuples are similar!

e = ("hello", "world")
e = e + ("!",)
print(e)
('hello', 'world', '!')

Exercise 2

Write a function ex2 that takes a single tuple argument, and returns a new tuple with the same contents, but with an additional item: an integer 9.

  • ex2((2, 3)) returns (2, 3, 9)
  • ex2(("A", "B", "C")) returns ("A", "B", "C", 9)

Submit as ex2.py.

Tuple Assignment

The assignment operator = works with tuples on both sides

x, y = ("A", 3)
print(x)
print(y)
A
3
  • Parentheses are optional:
x, y, z = 5, 6, 7
  • Tuple assignment sometimes called “multiple assignment”

Tuple Return

Tuple assignment used with function return:

def num_sq(x):
  return x, x**2

a, b = num_sq(5)
print(a)
print(b)
5
25

This does not work with lists!

Why Tuples?

  • Immutable
    • Never worry about modifying a list you didn’t mean to modify
  • Fast
    • Tuples run faster than lists
    • We don’t care about speed in CSCI 1012
    • We care about speed in the real world
  • Returns, assignment

Files

  • “Real” programming involves working with files
    • You probably know: MS Word, Excel, Images, Text
    • You probably don’t know: CSV, JSON, Protobuf, YAML…
  • We can read, work with, and write files in Python!

Opening Files

You will never need to memorize this:

with open(filename, "r") as f:
  contents = f.read()
# contents is now a string you can work with

Opening File: Example

File contents:

some_numbers.txt
3 4 5 6
7 8 9 10

Python script:

read_file.py
with open("some_numbers.txt", "r") as f:
  contents = f.read()
print(contents)
print(type(contents))
3 4 5 6
7 8 9 10
<class 'str'>

🚨 Writing Files 🚨

⚠️ ⚠️ WARNING ⚠️ ⚠️
❗ This can erase things on your computer! ❗


You will never need to memorize this:

write_file.py
string_to_write = "this will get written to the file"
with open("some_numbers.txt", "w") as f:
  print(string_to_write, file=f)

⚠️ If there is existing content in the some_numbers.txt file, it will be erased and replaced by the new content. ⚠️

Path

Exercise 3

  • Write a function ex3 that reads a file numbers.txt.
  • The numbers.txt file should be in the same path as the ex3.py file.
  • The file will contain positive integers separated by spaces, on several lines.

Your function should return the largest of these integers. For example, calling it on numbers.txt with these contents:

numbers.txt
1 4 5 24
2 5 6 25
101 102 6

should return int 102. Submit as ex3.py.