L-5
Objects, Collections, and Files
ENGG1330
Computer Programming I
L4 Survey Results
String method output correct
37%
Function output correct
65%
Slice output correct
76%
Not yet confident or need help
59%
421 submissions · 403–404 answers to these questions
List Assignment
inventory = ['key']backup = inventoryinventory[0] = 'map'print(backup)Will backup contain
key or map?
key or map?
Predict first, then follow the references
inventory refers to this list
Assignment adds a reference to the same list
The list changes; both names still refer to it
Value and Identity
x = ['key']y = ['key']print(x == y)== compares values
These two lists contain the same item
x = ['key']y = ['key']print(id(x))print(id(y))id(x) identifies the object
Equal values, different objects: their IDs differ
x = ['key']y = xprint(x is y)is checks for the same object
Both names refer to the same list
Diagram labels O1, O2, … stand for objects
They are not the numbers returned by id()
Comparing Objects
first = [10, 20]second = [10, 20]alias = firstprint(first == second)print(first is second)print(first is alias)Two list objects; one additional name
== compares these lists by contents: True
first is second: False
first is alias: True
O1, O2, …: symbolic object IDs
Shared References
first = ['key']alias = firstbefore = id(first)alias[0] = 'map'print(first, alias)print(id(first) == before)Mutation changes an existing object
The same object remains: id(first) == before
O1, O2, …: symbolic object IDs
Rebinding
first = ['key']alias = firstfirst = ['map']print(first)print(alias)print(first is alias)Rebinding changes which object a name refers to
Only first moves to the new object O2
O1, O2, …: symbolic object IDs
String Immutability
word = 'door'other = wordword.upper()print(word, other)word = word.upper()print(word, other)Predict both output lines
Which statement changes
what word refers to?
Which statement changes
what word refers to?
Explain your prediction before Run
O1, O2, …: symbolic object IDs
Tuples
ENGG1330
Computer Programming I
Single-Item Tuples
number = (0)packed = (0,)also_packed = 0,print(type(number))print(type(packed))print(packed, also_packed)Tuple Concatenation
values = (1, 2)previous = valuesvalues += (3,)print(previous, values)print(values is previous)Two names refer to the same tuple
values moves to O2; previous keeps O1
O1, O2, …: symbolic object IDs
List Augmentation
values = [1, 2]previous = valuesvalues += [3]print(previous, values)print(values is previous)Two names refer to the same list
O1 changes; both references stay
O1, O2, …: symbolic object IDs
Lists and Tuples: Check
a = [1]b = ax = (1,)y = xa += [2]x += (2,)print(b)print(y)Predict b and y
Why does the same +=
give different results?
Why does the same +=
give different results?
Explain using objects and references
O1, O2, …: symbolic object IDs
Function Returns
def show_product(x, y): print(x * y)def multiply(x, y): return x * yresult = show_product(3, 4)print(result)result = multiply(3, 4)print(result)print shows output
return sends a value to the caller
| After show_product(3, 4) | Value |
|---|---|
| Output on screen | 12 |
| result | None |
There is no return in this function
print(result) displays None
The screen now shows 12, then None
| After multiply(3, 4) | Value |
|---|---|
| Returned value | 12 |
| result | 12 |
The final print displays 12
Packing and Unpacking
point = (1, 2, 3)x, y, z = pointprint(x, y, z)Packing groups values in one tuple
| Tuple Item 0 | Tuple Item 1 | Tuple Item 2 |
|---|---|---|
| 1 | 2 | 3 |
Unpacking assigns items to names, in order
| x | y | z |
|---|---|---|
| 1 | 2 | 3 |
Use one target per item
() is an empty tuple
An iterable supplies items one at a time
Tuples, strings, and ranges are iterable
Sequence Unpacking
x, y, z = (1, 2, 3)a, b, c = range(3)first, second = 'Hi'print(x, y, z)print(a, b, c)print(first, second)Returning Multiple Values
def quotient_remainder(a, b): return a // b, a % bresult = quotient_remainder(17, 5)print(result)quotient, remainder = resultprint(quotient, remainder)Unpacking Errors
x, y = (1, 2, 3)print(x, y)Unpacking in Loops
rows = ()for i in range(3): rows += ((i, i * 2, i * 3),)print(rows)for x, y, z in rows: print(x, y, z)Record Summary: Plan
values = (2, 5, 4)names = ('Ada', 'Ben')print(min(values))print(max(values))print(len(names))min finds the smallest value; max finds the largest
| Value | Name |
|---|---|
| 2 | Ada |
| 5 | Ben |
| 4 | Ada |
Collect every value, but each name only once
Three records give two distinct names
The three summary results
| Minimum | Maximum | Names |
|---|---|---|
| 2 | 5 | 2 |
Assume at least one record
Record Summary: Code
def summarize(records): values, names = (), () for value, name in records: values += (value,) if name not in names: names += (name,) return min(values), max(values), len(names)records = ((2, 'Ada'), (5, 'Ben'), (4, 'Ada'))print(summarize(records))Unpacking Arguments
def show(x, y, z): print(x, y, z)point = (1, 2, 3)show(point)show(point) passes one tuple
The function needs three arguments
def show(x, y, z): print(x, y, z)point = (1, 2, 3)show(1, 2, 3)show(point[0], point[1], point[2])show(*point)*point unpacks its items into separate arguments
All three calls supply the same values
Point Displacement
def difference(x1, y1, z1, x2, y2, z2): return x2 - x1, y2 - y1, z2 - z1start = (1, 2, 3)end = (4, 6, 8)print(difference(*start, *end))Swapping Values
x, y = 3, 8temporary = xx = yy = temporaryprint(x, y)x, y = y, xprint(x, y)Tuple Immutability
pair = ([1, 2], [3, 4])pair[0] = [9, 9]print(pair)Lists Inside Tuples
pair = ([1, 2], [3, 4])pair[0][0] = 9print(pair)Tuple slots refer to the inner lists
The inner list changes; tuple slots stay
O1, O2, …: symbolic object IDs
List Operations
ENGG1330
Computer Programming I
Appending Items
values = [1, 2]values.append([3, 4])print(values)print(len(values))Start with two outer items
One new outer slot: len(values) is 3
O1, O2, …: symbolic object IDs
Extending Lists
values = [1, 2]values.extend([3, 4])print(values)print(len(values))Start with the same two outer items
len(values) is 4; no inner list is added
O1, O2, …: symbolic object IDs
List Concatenation
values = [1, 2]alias = valuesvalues = values + [3]print(values, alias)print(values is alias)Both names initially reach O1
+ creates O2; assignment redirects values
O1, O2, …: symbolic object IDs
Deleting and Removing
values = [4, 7, 4, 9]del values[1]print(values)values.remove(4)print(values)del values[1] selects index 1
After deletion, later items shift left
remove(4) selects the first matching value
Only the first 4 is removed
O1, O2, …: symbolic object IDs
Sorting in Place
values = [3, 1, 2]alias = valuesresult = values.sort()print(values, alias)print(result)Two names share the unsorted list
sort() changes O1; both names see it
The returned value is None
O1, O2, …: symbolic object IDs
Creating a Sorted List
values = [3, 1, 2]ordered = sorted(values)print(values)print(ordered)print(values is ordered)Start with the original ordering
sorted() returns O2; O1 keeps its order
O1, O2, …: symbolic object IDs
Copying with Slices
original = [1, 2]copied = original[:]copied[0] = 9print(original, copied)print(original is copied)A slice creates a new outer list O2
Replacing O2’s item leaves O1 unchanged
O1, O2, …: symbolic object IDs
Shallow Copies
original = [[1, 2], [3, 4]]copied = original[:]copied[0][1] = 9print(original)print(original is copied)print(original[0] is copied[0])O1 contains references to O3 and O4
O2 gets copies of the same references
Both outer lists reach the changed O3
O1, O2, …: symbolic object IDs
Nested References
a = [1, 2]b = [3, 4]c = [1, a, b]d = [a, c]a[1] = 42print(c)print(d[0] is c[1])c[1] and d[0] both refer to O1
a[1] changes the shared O1
O1, O2, …: symbolic object IDs
Rebinding Nested Lists
a = [1, 42]c = [1, a, [3, 4]]d = [a, c]a = [5, 6]d[0] = 1c[1][1] = 3print(a)print(d)Start with the same shared inner list
a moves to the new object O5
d[0] becomes 1; c[1] still reaches O1
c[1][1] changes O1, not O5
O1, O2, …: symbolic object IDs
Shallow Copies: Check
a = [[1], [2]]b = a[:]b[0].append(9)print(a)b[0] = [7]print(a, b)Predict both output lines
Which statement changes the shared inner list?
Removal During Iteration
def remove_blocked(values, blocked): for value in values: if value in blocked: values.remove(value)values = [1, 2, 3, 4]blocked = [1, 2]remove_blocked(values, blocked)print(values)Next visits index 0: value 1
After removing 1, Next advances to index 1
Visit 3, then advance to index 2
Visit 4, then finish: 2 was never checked
Iteration over a Copy
def remove_blocked(values, blocked): for value in values[:]: if value in blocked: values.remove(value)values = [1, 2, 3, 4]blocked = [1, 2]remove_blocked(values, blocked)print(values)Iterate over a copy; remove from the original
Remove 1; the copy still has 2 at index 1
Remove 2; the next copied item is 3
Visit 3 and 4; both remain in the original
List Comprehensions
ENGG1330
Computer Programming I
Building a List
squares = []for number in range(5): squares.append(number * number)print(squares)Start with an empty result list
Append 0; the list now has 1 item
Append 1; the list now has 2 items
Append 4; the list now has 3 items
Append 9; the list now has 4 items
Append 16; the list now has 5 items
Comprehension Syntax
squares = [number * number for number in range(5)]print(squares)numbers = [number for number in range(5)]print(numbers)Filtering with Comprehensions
evens = [n for n in range(8) if n % 2 == 0]inverses = [1 / n for n in range(-2, 3) if n != 0]print(evens)print(inverses)Building a Filtered List
values = [1, 2, 3, 4]blocked = [1, 2]kept = [value for value in values if value not in blocked]print(kept)print(values)Comprehensions: Check
result = [n * n for n in range(6) if n % 2 == 1]
Write the equivalent loop, then predict the result
The random Module
import randomrandom.seed(7)print(random.randint(1, 6))print(random.choice(('pass', 'retry')))A module groups reusable code
import random makes its functions available
seed(7) sets a repeatable starting state
The dot selects a function from the module
randint(1, 6) chooses an integer from 1 to 6
Both limits are included
choice selects one item from a sequence
Run twice and compare the outputs
Random Samples
import randomrandom.seed(7)rolls = [random.randint(1, 6) for roll in range(5)]print(rolls)Text Files
ENGG1330
Computer Programming I
Opening and Closing Files
with open('demo.txt', 'w') as file: print('File is open')print('The block has ended')open opens a file
'w' creates or replaces it; as file names the file object
Indented statements run inside the with block
The file is open here
Leaving the block closes the file
'r' opens an existing file for reading
Later examples add encoding='utf-8'
This selects the text encoding
File Methods
with open('demo.txt', 'w') as file: file.write('key\nmap\n')with open('demo.txt', 'r') as file: print(repr(file.read()))with open('demo.txt', 'r') as file: for line in file: print(repr(line))write(text) writes a string
\n separates the two lines
read() returns all remaining text
repr makes newline characters visible
A file is iterable
for line in file: reads one line at a time
Each line can include its newline
Writing and Reading Text
with open('demo_ids.txt', 'w', encoding='utf-8') as file: file.write('1001\n1002\n1003\n')with open('demo_ids.txt', 'r', encoding='utf-8') as file: text = file.read()print(repr(text))Reading Numbers
with open('demo_ids.txt', 'w', encoding='utf-8') as file: file.write('1001\n1002\n1003\n')ids = ()with open('demo_ids.txt', 'r', encoding='utf-8') as file: for line in file: ids += (int(line.strip()),)print(ids)print(len(ids), ids[1])Saving Simulated Outcomes
import randomrandom.seed(7)ids = (1001, 1002, 1003)with open('demo_results.txt', 'w', encoding='utf-8') as file: for identifier in ids: outcome = random.choice(('pass', 'retry')) file.write(f'{identifier},{outcome}\n')with open('demo_results.txt', 'r', encoding='utf-8') as file: print(file.read(), end='')L5 Survey
Complete the survey during the lecture
L5 Coding Challenges
Work independently on the four challenges