Day 4 – Lesson 14
Lists
Python Mini-Course
University of Oklahoma
Department of Psychology
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Python Mini-Course: Day 4 – Lesson 14
5/02/09
Lesson objectives
1. Describe the characteristics of the
list data structure in Python
2. Perform basic operations with lists
including creation, concatenation,
repetition, slicing, and traversing
3. Use string methods that require lists
(join, split)
4. Use lists in functions
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Python Mini-Course: Day 4 – Lesson 14
5/02/09
The list data structure
 In Python, a list is a mutable
sequence of values
 Each value in the list is an element
or item
 Elements can be any Python data
type
 Lists can mix data types
 Elements can be nested lists
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Python Mini-Course: Day 4 – Lesson 14
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Creating lists
numbers = [1, 2, 3, 4]
print numbers
cheeses = ['swiss', 'cheddar',
'ricotta', 'gouda']
print cheeses
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Python Mini-Course: Day 4 – Lesson 14
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Creating lists
mixed = [1, 'a', 3.45]
print mixed
single = ['z']
print single, type(single)
empty = []
print empty
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Python Mini-Course: Day 4 – Lesson 14
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Repeating a list
 Use the * operator:
meat = ['spam']*4
print meat
print [1, 2, 3]*3
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Python Mini-Course: Day 4 – Lesson 14
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List indexing
 Elements within a list are indexed
(see Lesson 10)
print cheeses[0]
 Lists are mutable
cheeses[0] = 'Feta'
print cheeses
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Python Mini-Course: Day 4 – Lesson 14
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Slicing a list
 Like strings and other sequences,
lists can be sliced
print cheeses[1:4]
print cheeses[:2]
print cheeses[2:]
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Python Mini-Course: Day 4 – Lesson 14
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Changing a slice
roster = ['Meghan', 'Tricia', 'Juan',
'Alton', 'Darrel', 'Jen']
print roster
roster[1:3] = ['Sam', 'Kerri']
print roster
roster[3:5] = ['Tayla']
print roster
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Python Mini-Course: Day 4 – Lesson 14
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Inserting elements
 Slice notation
roster[2:2] = ['Dana', 'Ryan']
print roster
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Python Mini-Course: Day 4 – Lesson 14
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Deleting elements
 Set slice to empty list
roster[3:5] = []
print roster
 The del keyword
del roster[2:3]
print roster
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Python Mini-Course: Day 4 – Lesson 14
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The insert and append methods
 The insert method
roster.insert(2,'Jakob')
print roster
 The append method
roster.append('Tonya')
print roster
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Python Mini-Course: Day 4 – Lesson 14
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The extend method
 Adds a list to the end of an
existing list
adds = ['Ian', 'Stacie']
roster.extend(adds)
print roster
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Python Mini-Course: Day 4 – Lesson 14
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Extending a list
 Can also use += operator
roster += ['Anya']
print roster
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Python Mini-Course: Day 4 – Lesson 14
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Using the + operator
a = [1, 2, 3]
b = [4, 5, 6]
c = a + b
print a, b, c
*The + operator returns a new list that
is a concatenation of two lists
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Python Mini-Course: Day 4 – Lesson 14
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Note on list operations
 Be careful when using the +
operator and append method
 Try this:
d = c + 7
 Or this
c.append(b)
print c
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Python Mini-Course: Day 4 – Lesson 14
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List assignment and aliasing
a = [1, 2, 3, 4]
b = a
c = a[:]
a[2] = 9
print a, b, c
*The slice operator returns a copy of a list
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Python Mini-Course: Day 4 – Lesson 14
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Other list methods
roster.sort()
print roster
roster.reverse()
print roster
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Python Mini-Course: Day 4 – Lesson 14
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Other list methods
print roster.index('Tonya')
print roster.index('Tonya', 2, 5)
print roster.count('Sam')
roster.remove('Sam')
print roster
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Python Mini-Course: Day 4 – Lesson 14
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The join string method
 Concatenates a sequence of
strings into a single string with
sep inserted between each item.
 Syntax: sep.join(list)
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Python Mini-Course: Day 4 – Lesson 14
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The split string method
 Returns a list of words from a
string using sep as the delimiter
string
 Syntax: sep.split(list)
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Python Mini-Course: Day 4 – Lesson 14
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Example: join_split.py
t = ['pining', 'for', 'the', 'fjords']
delimiter = '_'
s = delimiter.join(t)
print s
u = s.split(delimiter)
print u
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Python Mini-Course: Day 4 – Lesson 14
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Example
print ''.join(t)
print ' '.join(t)
print '\t'.join(t)
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Python Mini-Course: Day 4 – Lesson 14
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Traversing a list
for index in range(len(roster)):
print roster[index]
for student in roster:
print student
for index, student in enumerate(roster):
print index, student
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Python Mini-Course: Day 4 – Lesson 14
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Traversing a list
 What does this do?
empty = []
for x in empty:
print x
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Python Mini-Course: Day 4 – Lesson 14
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Nested lists
nested = [[1,2,3],[4,5,6],[7,8,9]]
print nested
print nested[0]
print nested[0][1]
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Python Mini-Course: Day 4 – Lesson 14
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Traversing nested lists
for i in range(len(nested)):
for j in range(len(nested[i])):
print nested[i][j]
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Python Mini-Course: Day 4 – Lesson 14
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Traversing nested lists
for nest in nested:
for item in nest:
print item
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Python Mini-Course: Day 4 – Lesson 14
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Using lists: cumulate.py
def cumulate(seq):
c_sum = 0
for item in seq:
c_sum += item
return c_sum
a = [12, 78, 32, 82]
s = cumulate(a)
print s
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Python Mini-Course: Day 4 – Lesson 14
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Returning lists from functions:
only_upper.py
def only_upper(t):
res = []
for s in t:
if s.isupper():
res.append(s)
return res
text = 'Bold cOlOrs Make for Easy Reading'
secret = only_upper(text)
print secret
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Python Mini-Course: Day 4 – Lesson 14
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Modifying lists in functions
 In Python, arguments are passed
by reference
 The parameter in the function is an
alias for the argument that was
passed in
 If a mutable object is changed inside
the function, it is also changed
outside the function
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Python Mini-Course: Day 4 – Lesson 14
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Example: byref.py
def change(seq):
print 'Passed in: ' + str(seq)
seq.append('new item')
print 'Changed to: ' + str(seq)
original = [1, 2, 3]
print original
change(original)
print original
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Python Mini-Course: Day 4 – Lesson 14
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Example: byref2.py
def change(seq):
print 'Passed in: ' + str(seq)
seq.append('new item')
print 'Changed to: ' + str(seq)
new_seq = ['created','in','function']
print 'New seq: ' + str(new_seq)
original = [1, 2, 3]
new_seq = ['outside','the','function']
print original
change(original)
print original
print new_seq
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Python Mini-Course: Day 4 – Lesson 14
5/02/09
Suggested exercises
 Exercise 10.5 – Solving the
"Birthday Paradox" by a Monte
Carlo simulation
 Exercise 10.6 – Removing
duplicates from a list
 Exercise 10.8 – Bisection search
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Python Mini-Course: Day 4 – Lesson 14
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