Outlines

• Control flows
• Loops
• Basic string operators
• file operations
• exceptions
• datetime
• more on python data structure
  • list
  • dictionary
  • tuple

Control flows

num = input("Enter a number: ")
if int(num) > 0:
    print(f"{num} is positive")

• colon(:) at the end of the if line
• indent (e.g., 2 (or 4) whitespaces) before the chunk of code to be executed
• In Python if control folow, we don’t have parentheses like in R. Indentation is used to determine the end of the code chunk.

Indentation

• Indentation serves another purpose other than code readability
• Python treats the statements with the same indentation level (statements with an equal number of whitespaces before them) as a single code block.
• Commonly used indent
  • 2 whitespaces
  • 4 whitespaces
  • 1 tab (not recommended)
• This rule of identation is used for flow control, loops, functions etc.

if else elif

num = input("Enter a number: ")
if int(num) > 0:
    print(f"{num} is positive")
else:
    print(f"{num} is not positive")

num = input("Enter a number: ")
if int(num) > 0:
    print("The number is positive")
elif int(num) < 0:
    print("The number is negative")
else:
    print("The number is zero")

if else same line

• original

number = input("Please enter a number: ")
if int(number) % 2 == 0:
    print("even")
else:
    print("odd")

• one line version

number = input("Please enter a number: ")
print("even") if int(number) % 2 == 0 else print("odd")

True or False conditions

1>2

## False

4 == 5

## False

"ap" in "apple"

## True

"apple" in ["apple", "orange"]

## True

True and False

## False

True or False

## True

not True

## False

True or False conditions

• we could use < (or >) to connect a series of comparisons

a = 4
b = 6
c = 9

a < b and b < c

## True

a < b < c

## True

a < c > b

## True

match (available for python >= 3.10)

• To select one cases from multiple choices

status = 400    

match status:
    case 400:
        print("Bad request")
    case 404:
        print("Not found")
    case 418:
        print("I'm a teapot")
    case _:
        print("Something's wrong with the internet")

## Bad request

for loops

words = ["cat", "dog", "gator"]
for w in words:
     print(w)

## cat
## dog
## gator

words = ["cat", "dog", "gator"]
for w in words:
     print(f"{w} has {len(w)} letters in it.")

## cat has 3 letters in it.
## dog has 3 letters in it.
## gator has 5 letters in it.

range() function

for i in range(3):
    print(i)

## 0
## 1
## 2

• range(n) creates an iterable object
• list(iterable) convert an iterable to a list
• The range(n) is exclusive, it doesn’t include the last number n. 
• It creates the sequence of numbers from start to stop -1.
• For example, list(range(5)) will produce [0, 1, 2, 3, 4]

list(range(3))

## [0, 1, 2]

list(range(3,7))

## [3, 4, 5, 6]

range() function

• range with step size rather than 1.

list(range(3,8,2))

## [3, 5, 7]

list(range(7,2,-2))

## [7, 5, 3]

• range over a list

words = ["cat", "dog", "gator"]
for i in range(len(words)):
     print(i, words[i])

## 0 cat
## 1 dog
## 2 gator

break

for num in range(1, 10):
  if num % 5 == 0:
    print(f"{num} can be divided by 5")
    break
  print(f"{num} cannot be divided by 5")

## 1 cannot be divided by 5
## 2 cannot be divided by 5
## 3 cannot be divided by 5
## 4 cannot be divided by 5
## 5 can be divided by 5

continue

for num in range(1, 10):
  if num % 5 == 0:
    continue
  print(f"{num} cannot be divided by 5")

## 1 cannot be divided by 5
## 2 cannot be divided by 5
## 3 cannot be divided by 5
## 4 cannot be divided by 5
## 6 cannot be divided by 5
## 7 cannot be divided by 5
## 8 cannot be divided by 5
## 9 cannot be divided by 5

pass

• In python, pass is the null statement.
• It is just a placeholder for the functionality to be added later.
• Pass does nothing.

sequence = {'p', 'a', 's', 's'}
for val in sequence:
    pass

a = 33
b = 200

if b > a:
  pass

while loop

num = 1
while num<10:
  if num % 5 == 0:
    print(f"{num} can be divided by 5")
    break
  print(f"{num} cannot be divided by 5")
  num+=1

## 1 cannot be divided by 5
## 2 cannot be divided by 5
## 3 cannot be divided by 5
## 4 cannot be divided by 5
## 5 can be divided by 5

num = 0
while num<10:
  num+=1
  if num % 5 == 0:
    continue
  print(f"{num} cannot be divided by 5")

## 1 cannot be divided by 5
## 2 cannot be divided by 5
## 3 cannot be divided by 5
## 4 cannot be divided by 5
## 6 cannot be divided by 5
## 7 cannot be divided by 5
## 8 cannot be divided by 5
## 9 cannot be divided by 5

Basic string operators

• find
• index
• count
• join
• split
• lower
• upper
• title
• replace
• strip

find

title = "I love programming basics for Biostatistics!"
title.find("I")

## 0

title.find("love")

## 2

title.find("o")

## 3

title.find("o", 4) ## starting searching index is 4

## 9

title.find("XX")

## -1

• find() and index() are identical except when not found
  • find() produces -1
  • index() produces an error

title.index("love")
title.index("XX")

pattern detection

title = "I love programming basics for Biostatistics!"
"love" in title

## True

"computing" in title

## False

"XX" in title

## False

title.endswith("computing!")

## False

title.startswith("I love")

## True

title.count("l")

## 1

join

seq = ["1", "2", "3", "4", "5"]
sep = "+"
sep.join(seq)

## '1+2+3+4+5'

"".join(seq)

## '12345'

dirs =( "", "usr", "bin", "env")
"/".join(dirs)

## '/usr/bin/env'

sep = "+"
print("C:" + "\\".join(dirs)) ## single \ has special meaning: treating special symbol as regular symbol 

## C:\usr\bin\env

split

• reverse operator of join.

longSeq = "1+2+3+4+5"
longSeq.split("+")

## ['1', '2', '3', '4', '5']

longSeq.split("3")

## ['1+2+', '+4+5']

"Using the default value".split()

## ['Using', 'the', 'default', 'value']

lower, upper, title

sentence = "I like programming basics for Biostatistics!"
sentence.lower()

## 'i like programming basics for biostatistics!'

sentence.upper()

## 'I LIKE PROGRAMMING BASICS FOR BIOSTATISTICS!'

sentence.title()

## 'I Like Programming Basics For Biostatistics!'

sentence.islower()
sentence.isupper()
sentence.istitle()

strip

• removes any leading (whitespace at the beginning) and trailing (whitespace at the end) characters.
• whitespace is the default leading character to remove
• internal whitespace is kept

a = "   internal   whitespace is kept     "
a.strip()

## 'internal   whitespace is kept'

b = "*** SPAM * for * everyone!!! ***"
b.strip(" *!")

## 'SPAM * for * everyone'

• also works for new line

c = "\na\nb\n\n\nc\n\n"
c.strip()

## 'a\nb\n\n\nc'

replace

a = "This is a cat!"
a.replace("This", "That")

## 'That is a cat!'

a.replace("is", "eez")

## 'Theez eez a cat!'

file operation (read)

https://caleb-huo.github.io/teaching/data/misc/my_file.txt

• open file, display, and close (release memory)

file = open("my_file.txt")
contents = file.read()
print(contents)

## Hello, my name is Caleb. Hello World!
## I like computing

file.close()

• Alternative approach without closing step

with open("my_file.txt") as file:
    contents = file.read()
    print(contents)

## Hello, my name is Caleb. Hello World!
## I like computing

file operation (read)

• readlines()
  • read multilple lines,
  • save the result in a list
    • each element of the list contains a line

myfile = "my_file.txt"
with open(myfile) as file:
    lines = file.readlines()

for aline in lines:
    print(aline.strip())

## Hello, my name is Caleb. Hello World!
## I like computing

file operation (write)

• write to file (overwrite original file)

with open("new_file.txt", mode="w") as file:
    file.write("I like python!")

## 14

• append to file (append at the end of the original file)

with open("new_file.txt", mode="a") as file:
    file.write("We like python!")

## 15

Exceptions

• FileNotFound

with open("a_file.txt") as file:
    file.read()

• IndexError

fruit_list = ["Apple", "Banana", "Pear"]
fruit_list[3]

• TypeError

text = "abc"
print(text + 5)

• raise an error

raise TypeError("This is an error that I made up!")

Handle exceptions

• The errors (exceptions) are handled by except.
• The program will keep executing.

try:
    file = open("a_file.txt")
    print(1 + "2")
except FileNotFoundError:
    print("Catch FileNotFoundError")
except TypeError as error_message:
    print(f"Here is the error: {error_message}.")
else:
    content = file.read()
    print(content)
finally: ## will happen no matter what happens
    file.close()
    print("File was closed.")

## Catch FileNotFoundError
## File was closed.

datetime

• The datetime module supplies classes for manipulating dates and times.

import datetime as dt
now = dt.date.today() ## date only
now.year

## 2023

now.month

## 10

now.day

## 12

# now.weekday()

birthday = dt.date(1995, 7, 31)
age = now - birthday
age.days

## 10300

datetime

now = dt.datetime.now()
now.year

## 2023

now.month

## 10

now.day

## 12

now.hour

## 13

now.minute

## 18

now.second

## 59

now.microsecond

## 756676

now.weekday()

## 3

datetime

now = dt.datetime.now()

print(f'{now:%Y-%m-%d %H:%M}')

## 2023-10-12 13:18

more on python data structure

• list
• dictionary
• tuple

list: creation and assignment

• create a list from a string

list("Hello")

## ['H', 'e', 'l', 'l', 'o']

• create a list directly

x = [1, 1, 1]
x

## [1, 1, 1]

• change element

x = [1, 2, 3]
x[1] = 0
x

## [1, 0, 3]

list: deletion and slice assignment

names = ["Alice", "Beth", "Carl", "Dan", "Emily"]
names

## ['Alice', 'Beth', 'Carl', 'Dan', 'Emily']

del names[2]
names

## ['Alice', 'Beth', 'Dan', 'Emily']

• slice assignment

names = list("Lucas")
names[3:] = list("ky")
names

## ['L', 'u', 'c', 'k', 'y']

"".join(names)

## 'Lucky'

• slice assignment can be unequal length

names = list("Lucas")
names[1:] = list("emonade")
"".join(names)

## 'Lemonade'

• slice assignment can be used as insertion or deletion

numbers = [1, 5]
numbers[1:1] = [2, 3, 4]
numbers

## [1, 2, 3, 4, 5]

numbers = list(range(1,6))
numbers

## [1, 2, 3, 4, 5]

numbers[1:4] = []
numbers

## [1, 5]

list: append and count

• append

alist = [0,1,2]
alist.append(3)
alist

## [0, 1, 2, 3]

• count

asentence = "to be or not to be"
alist = asentence.split()
alist.count("to")

## 2

x = [[1,2], 1, 2, 1, [2, 1, [1,2]]]
x.count(1)

## 2

x.count([1,2])

## 1

list: extend

• extend (recommended for efficiency and readibility)

a = [0,1,2]; b = [3,4,5]
a.extend(b)
a

## [0, 1, 2, 3, 4, 5]

• direct +

a = [0,1,2]; b = [3,4,5]
a + b

## [0, 1, 2, 3, 4, 5]

a

## [0, 1, 2]

• slice assignment

a = [0,1,2]; b = [3,4,5]
a[len(a):] = b
a

## [0, 1, 2, 3, 4, 5]

list: index

• index: will return the first match

asentence = "to be or not to be"
alist = asentence.split()
alist

## ['to', 'be', 'or', 'not', 'to', 'be']

alist.index("to")

## 0

alist.index("not")

## 3

alist[3]

## 'not'

alist.index("XX")

list: insert

• insert

alist = [1,2,3,5,6]
alist.insert(3, "four")
alist

## [1, 2, 3, 'four', 5, 6]

• insert with slice assignment

alist = [1,2,3,5,6]
alist[3:3] = ["four"]
alist

## [1, 2, 3, 'four', 5, 6]

list: pop

• pop: return the last element of a list
  • opposite of append

x = list(range(10))
x.pop()

## 9

x

## [0, 1, 2, 3, 4, 5, 6, 7, 8]

x.pop()

## 8

x

## [0, 1, 2, 3, 4, 5, 6, 7]

list: remove

• remove

asentence = "to be or not to be"
alist = asentence.split()
alist

## ['to', 'be', 'or', 'not', 'to', 'be']

alist.remove("to")
alist

## ['be', 'or', 'not', 'to', 'be']

alist.remove("XX")

• compare pop and remove
  • remove has no return value, and remove the first appearance of certain value
  • pop has return value, and pop up the last element of a list

list: reverse and sort

• reverse

x = ["a", "b", "c"]
x.reverse()
x

## ['c', 'b', 'a']

• sort: sort method has no return value (in-place operator)

x = [5, 3, 4]
x.sort() 
x

## [3, 4, 5]

y = ["b", "c", "a"]
y.sort()
y

## ['a', 'b', 'c']

• no return values:

x = [5, 3, 4]
y = x.sort() 
print(y)

## None

• with return values:

x = [5, 3, 4]
y = sorted(x) 
print(y)

## [3, 4, 5]

list: sort

• sort

x = [5, 3, 4]
y = x ## x and y are pointing to the same list
y.sort() 

print(x)

## [3, 4, 5]

print(y)

## [3, 4, 5]

x = [5, 3, 4]
y = x[:] ## y is a slice assignment of x, thus a new variable
y.sort() 

print(x)

## [5, 3, 4]

print(y)

## [3, 4, 5]

references and values

list: sort

• sort

x = ["aaa", "bb", "cccc"]
x.sort(key = len) 
x

## ['bb', 'aaa', 'cccc']

x = [5, 3, 4]
x.sort(reverse = True) 
print(x)

## [5, 4, 3]

dictionary: basic operator

• basic operator

phonebook = {"Alice": 2341, 
            "Beth": 4971,
            "Carl": 9401
}
phonebook

## {'Alice': 2341, 'Beth': 4971, 'Carl': 9401}

len(phonebook)

## 3

phonebook["Beth"]

## 4971

dictionary: update and delete

• update and delete

phonebook["Alice"] = 1358
phonebook

## {'Alice': 1358, 'Beth': 4971, 'Carl': 9401}

adict = {"Alice": 9572}
phonebook.update(adict)
phonebook

## {'Alice': 9572, 'Beth': 4971, 'Carl': 9401}

del phonebook["Carl"]
"Beth" in phonebook

## True

dictionary: clear

• clear

d = {}
d['name'] = "Amy"
d['age'] = 24
d

## {'name': 'Amy', 'age': 24}

d.clear()
d

## {}

why clear is useful

x = {}
y = x
x['key'] = 'value'
y

## {'key': 'value'}

x = {} ## now x points to a new value {}
y ## y points to the original value {'key': 'value'}

## {'key': 'value'}

x = {}
y = x
x['key'] = 'value'
y

## {'key': 'value'}

x.clear() ## clear the value x points to
y ## y still points to what x points to

## {}

references and values (part 2)

copy

• shallow copy
  • only the reference address of the object is copied

d = {}
d['username'] = "admin"
d['machines'] = ["foo", "bar"]
d

## {'username': 'admin', 'machines': ['foo', 'bar']}

c = d.copy()
c['username'] = "Alex" ## c['username'] points to a new value
print(c)

## {'username': 'Alex', 'machines': ['foo', 'bar']}

print(d)

## {'username': 'admin', 'machines': ['foo', 'bar']}

c['machines'].remove("bar") ## references don't change, the underlying values are changed.
print(c)

## {'username': 'Alex', 'machines': ['foo']}

print(d)

## {'username': 'admin', 'machines': ['foo']}

references and values (shallow copy)

copy

• deep copy:
  • will make a new copy of the values

from copy import deepcopy

d = {}
d['username'] = "admin"
d['machines'] = ["foo", "bar"]
d

## {'username': 'admin', 'machines': ['foo', 'bar']}

c = d.copy()
dc = deepcopy(d)
d['machines'].remove("bar") 
print(c)

## {'username': 'admin', 'machines': ['foo']}

print(dc)

## {'username': 'admin', 'machines': ['foo', 'bar']}

references and values (deep copy)

dictionary initialization: fromkeys

• create keys for an empty dictionary.

{}.fromkeys(["name", "age"])

## {'name': None, 'age': None}

• create keys for a dictionary

dict.fromkeys(["name", "age"])

## {'name': None, 'age': None}

• set default values

dict.fromkeys(["name", "age"], "unknown")

## {'name': 'unknown', 'age': 'unknown'}

dictionary: get

• get method is more flexible
• get is the same as indexing by keys when the key exists

d = {"name": "Amy", "age": 24}
d["name"]

## 'Amy'

d.get("name")

## 'Amy'

• get will return None when the key doesn’t exist

d["XX"]
d.get("XX")
d.get("XX", "No exist") ## set your own return value for get

dictionary: items

• items() return all items of the dictionary

phonebook = {"Alice": 2341, 
            "Beth": 4971,
            "Carl": 9401
}
phonebook

## {'Alice': 2341, 'Beth': 4971, 'Carl': 9401}

phonebook.items() ## this is an iterable

## dict_items([('Alice', 2341), ('Beth', 4971), ('Carl', 9401)])

list(phonebook.items())

## [('Alice', 2341), ('Beth', 4971), ('Carl', 9401)]

dictionary: loops

• can be used for looping a dictionary

it = phonebook.items()
for key, value in it:
    print(key +  "--> " + str(value))

## Alice--> 2341
## Beth--> 4971
## Carl--> 9401

• if you only want the value, not the keys

it = phonebook.items()
for _, value in it:
    print(str(value))

## 2341
## 4971
## 9401

• use key to iterate a dictionary for a loop

for key in phonebook:
    print(key +  "--> " + str(phonebook[key]))

## Alice--> 2341
## Beth--> 4971
## Carl--> 9401

• use values() method

phonebook.values() ## this is an iterable

## dict_values([2341, 4971, 9401])

list(phonebook.values())

## [2341, 4971, 9401]

for i in phonebook.values():
    print(i)

## 2341
## 4971
## 9401

dictionary: pop and popitem

• pop

phonebook = {"Alice": 2341, 
            "Beth": 4971,
            "Carl": 9401
}
phonebook.pop("Alice")

## 2341

phonebook

## {'Beth': 4971, 'Carl': 9401}

• popitem(): pop up the last item

phonebook = {"Alice": 2341, 
            "Beth": 4971,
            "Carl": 9401
}
phonebook.popitem()

## ('Carl', 9401)

phonebook

## {'Alice': 2341, 'Beth': 4971}

tuple: review basics

atuple = (0,1,2)
atuple += (3,4,5)
atuple

## (0, 1, 2, 3, 4, 5)

btuple = (0, 1, 1, ['I', 'like',  'python'])
btuple[3][0] = 'You'
print(btuple)

## (0, 1, 1, ['You', 'like', 'python'])

print(btuple.count(1))

## 2

print(btuple.index(['You', "like", 'python']))

## 3

Reference

• https://docs.python.org/3/index.html
• https://github.com/Apress/beginning-python-3ed