Strings in Python – miro.borodziuk.eu

1. Python strings are immutable sequences and can be indexed, sliced, and iterated like any other sequence, as well as being subject to the in and not in operators. There are two kinds of strings in Python:

one-line strings, which cannot cross line boundaries – we denote them using either apostrophes ('string') or quotes ("string")
multi-line strings, which occupy more than one line of source code, delimited by trigraphs:

'''
string
'''

'''

string

'''

"""
string
"""

"""

string

"""

2. The length of a string is determined by the len() function. The escape character (\) is not counted. For example:

print(len("\n\n"))

1	print(len("\n\n"))

2

x="\\\\"
print(len(x))

1 2	x="\\\\" print(len(x))

2

x="\\\"
print(len(x))

1 2	x="\\\" print(len(x))

x="\\\"
^
SyntaxError: unterminated string literal (detected at line 1)

\ escape only one character.

3. Strings can be concatenated using the + operator, and replicated using the * operator. For example:

asterisk = '*'
plus = "+"
decoration = (asterisk + plus) * 4 + asterisk
print(decoration)

asterisk = '*'

plus = "+"

decoration = (asterisk + plus) * 4 + asterisk

print(decoration)

outputs *+*+*+*+*.

4. The pair of functions chr() and ord() can be used to create a character using its codepoint, and to determine a codepoint corresponding to a character. Both of the following expressions are always true:

chr(ord(character)) == character
ord(chr(codepoint)) == codepoint

1 2	chr(ord(character)) == character ord(chr(codepoint)) == codepoint

5. Some other functions that can be applied to strings are:

list() – create a list consisting of all the string’s characters;
max() – finds the character with the maximal codepoint;
min() – finds the character with the minimal codepoint.

6. The method named index() finds the index of a given substring inside the string and raises an exception if the value is not found.

The syntax is as follows :
string.index(value, start, end) where:
– value [required] : the string/character to search for
– start [optional] : the index to start the search (default is 0)
– end [optional] : the index to end the search (default is the end of the string) – note that this index is excluded from the search.

    my_str = 'Luke !!\n'
    z = my_str.index('n')
    print(z)                # ValueError Exception
     
    x = my_str.index('L')
    print(x)                # 0
     
    y = my_str.index('!',4)
    print(y)                # 5
     
    w = my_str.index('!',1,5)
    print(w)                # ValueError Exception

my_str = 'Luke !!\n'

z = my_str.index('n')

print(z) # ValueError Exception

x = my_str.index('L')

print(x) # 0

y = my_str.index('!',4)

print(y) # 5

w = my_str.index('!',1,5)

print(w) # ValueError Exception

In the above question, my_str.index('n') will return the index of the first occurrence of the letter “n” in the string my_str, searching between the character at position 0 and the last character of the string (i.e. the whole string). In the string ‘Luke !!\n’ , '\n' is actually the code for a new line (note that escape character ‘\’); this is not representing the character ‘n’. So character 'n' is NOT in the string ‘Luke !!\n’ and my_str.index(‘n’) will raise an exception.

7. Some of the methods offered by strings are:

capitalize() – changes all string letters to capitals;

# Demonstrating the capitalize() method:
print('aBcD'.capitalize())
print("Alpha".capitalize())
print('ALPHA'.capitalize())
print(' Alpha'.capitalize())
print('123'.capitalize())
print("αβγδ".capitalize())

# Demonstrating the capitalize() method:

print('aBcD'.capitalize())

print("Alpha".capitalize())

print('ALPHA'.capitalize())

print(' Alpha'.capitalize())

print('123'.capitalize())

print("αβγδ".capitalize())

Abcd
Alpha
Alpha
alpha
123
Αβγδ

center() – centers the string inside the field of a known length;

# Demonstrating the center() method:
print('[' + 'alpha'.center(10) + ']')
print('[' + 'Beta'.center(2) + ']')
print('[' + 'Beta'.center(4) + ']')
print('[' + 'Beta'.center(6) + ']')

# Demonstrating the center() method:

print('[' + 'alpha'.center(10) + ']')

print('[' + 'Beta'.center(2) + ']')

print('[' + 'Beta'.center(4) + ']')

print('[' + 'Beta'.center(6) + ']')

[ alpha ]
[Beta]
[Beta]
[ Beta ]

The two-parameter variant of center() makes use of the character from the second argument, instead of a space. Analyze the example below:

print('[' + 'gamma'.center(20, '*') + ']')

1	print('[' + 'gamma'.center(20, '*') + ']')

[*******gamma********]

count() – counts the occurrences of a given character;

join() – joins all items of a tuple/list into one string;

# Demonstrating the join() method:
print(",".join(["omicron", "pi", "rho"]))

1 2	# Demonstrating the join() method: print(",".join(["omicron", "pi", "rho"]))

omicron,pi,rho

myTuple = ("John", "Peter", "Vicky")
x = "#".join(myTuple)
print(x)

myTuple = ("John", "Peter", "Vicky")

x = "#".join(myTuple)

print(x)

John#Peter#Vicky

lower() – converts all the string’s letters into lower-case letters;

# Demonstrating the lower() method:
print("SiGmA=60".lower())

1 2	# Demonstrating the lower() method: print("SiGmA=60".lower())

sigma=60

lstrip() – removes the white characters from the beginning of the string;

# Demonstrating the lstrip() method:
print("[" + " tau ".lstrip() + "]")

1 2	# Demonstrating the lstrip() method: print("[" + " tau ".lstrip() + "]")

[tau ]

The one-parameter lstrip() method does the same as its parameterless version, but removes all characters enlisted in its argument (a string), not just whitespaces:

print("www.cisco.com".lstrip("w."))

1	print("www.cisco.com".lstrip("w."))

cisco.com

rstrip() – removes the trailing white spaces from the end of the string;

do nearly the same as lstrips, but affect the opposite side of the string.

# Demonstrating the rstrip() method:
print("[" + " upsilon ".rstrip() + "]")
print("cisco.com".rstrip(".com"))

# Demonstrating the rstrip() method:

print("[" + " upsilon ".rstrip() + "]")

print("cisco.com".rstrip(".com"))

[ upsilon]

cis

strip() – removes the leading and trailing white spaces;

combines the effects caused by rstrip() and lstrip() – it makes a new string lacking all the leading and trailing whitespaces.

# Demonstrating the strip() method:
print("[" + "   aleph   ".strip() + "]")

1 2	# Demonstrating the strip() method: print("[" + " aleph ".strip() + "]")

[aleph]

txt = "     banana     "
x = txt.strip()
print("of all fruits", x, "is my favorite")

txt = " banana "

x = txt.strip()

print("of all fruits", x, "is my favorite")

of all fruits banana is my favorite

txt = ",,,,,rrttgg.....banana....rrr"
x = txt.strip(",.grt")
print(x)

txt = ",,,,,rrttgg.....banana....rrr"

x = txt.strip(",.grt")

print(x)

banana

split() – splits the string into a substring using a given delimiter;

# Demonstrating the split() method:
print("phi       chi\npsi".split())

1 2	# Demonstrating the split() method: print("phi chi\npsi".split())

['phi', 'chi', 'psi']

txt = "hello, my name is Peter, I am 26 years old"
x = txt.split(", ")
print(x)

txt = "hello, my name is Peter, I am 26 years old"

x = txt.split(", ")

print(x)

['hello', 'my name is Peter', 'I am 26 years old']

Note: the reverse operation can be performed by the join() method.

replace() – replaces a given substring with another;

# Demonstrating the replace() method:
print("www.netacad.com".replace("netacad.com", "pythoninstitute.org"))
print("This is it!".replace("is", "are"))
print("Apple juice".replace("juice", ""))

# Demonstrating the replace() method:

print("www.netacad.com".replace("netacad.com", "pythoninstitute.org"))

print("This is it!".replace("is", "are"))

print("Apple juice".replace("juice", ""))

www.pythoninstitute.org
Thare are it!
Apple

The three-parameter replace() variant uses the third argument (a number) to limit the number of replacements.

txt = "one one was a race horse, two two was one too."
x = txt.replace("one", "three", 2)
print(x)

txt = "one one was a race horse, two two was one too."

x = txt.replace("one", "three", 2)

print(x)

three three was a race horse, two two was one too.

find() – it looks for a substring and returns the index of first occurrence of this substring. For an argument containing a non-existent substring (it returns -1 then)

# Demonstrating the find() method:
print("Eta".find("ta"))
print("Eta".find("mma"))

# Demonstrating the find() method:

print("Eta".find("ta"))

print("Eta".find("mma"))

1
-1

rfind() – finds a substring starting from the end of the string; string.rfind(value, start, end)

# Demonstrating the rfind() method:
print("tau tau tau".rfind("ta"))
print("tau tau tau".rfind("ta", 9))
print("tau tau tau".rfind("ta", 3, 9))

# Demonstrating the rfind() method:

print("tau tau tau".rfind("ta"))

print("tau tau tau".rfind("ta", 9))

print("tau tau tau".rfind("ta", 3, 9))

8
-1
4

txt = "Mi casa, su casa."
x = txt.rfind("casa")
print(x)

txt = "Mi casa, su casa."

x = txt.rfind("casa")

print(x)

12

txt = "Hello, welcome to my world."
x = txt.rfind("e", 5, 10)
print(x)

txt = "Hello, welcome to my world."

x = txt.rfind("e", 5, 10)

print(x)

8

swapcase() – swaps the letters’ cases (lower to upper and vice versa)

# Demonstrating the swapcase() method:
print("I know that I know nothing.".swapcase())

1 2	# Demonstrating the swapcase() method: print("I know that I know nothing.".swapcase())

i KNOW THAT i KNOW NOTHING.

title() – makes the first letter in each word upper-case;

# Demonstrating the title() method:
print("I know that I know nothing. Part 1.".title())

1 2	# Demonstrating the title() method: print("I know that I know nothing. Part 1.".title())

I Know That I Know Nothing. Part 1.

upper() – converts all the string’s letter into upper-case letters.

# Demonstrating the upper() method:
print("I know that I know nothing. Part 2.".upper())

1 2	# Demonstrating the upper() method: print("I know that I know nothing. Part 2.".upper())

I KNOW THAT I KNOW NOTHING. PART 2.

2. String content can be determined using the following methods (all of them return Boolean values):

endswith() – does the string end with a given substring?

# Demonstrating the endswith() method:
if "epsilon".endswith("on"):
    print("yes")
else:
    print("no")

t = "zeta"
print(t.endswith("a"))
print(t.endswith("A"))
print(t.endswith("et"))
print(t.endswith("eta"))

# Demonstrating the endswith() method:

if "epsilon".endswith("on"):

print("yes")

else:

print("no")

t = "zeta"

print(t.endswith("a"))

print(t.endswith("A"))

print(t.endswith("et"))

print(t.endswith("eta"))

yes
True
False
False
True

startswith() – does the string begin with a given substring?

The startswith() method is a mirror reflection of endswith() – it checks if a given string starts with the specified substring.

# Demonstrating the startswith() method:
print("omega".startswith("meg"))
print("omega".startswith("om"))

# Demonstrating the startswith() method:

print("omega".startswith("meg"))

print("omega".startswith("om"))

False
True

isalnum() – does the string consist only of letters and digits?

# Demonstrating the isalnum() method:
print('lambda30'.isalnum())
print('lambda'.isalnum())
print('30'.isalnum())
print('@'.isalnum())
print('lambda_30'.isalnum())
print(''.isalnum())

# Demonstrating the isalnum() method:

print('lambda30'.isalnum())

print('lambda'.isalnum())

print('30'.isalnum())

print('@'.isalnum())

print('lambda_30'.isalnum())

print(''.isalnum())

True
True
True
False
False
False

t = 'Six lambdas'
print(t.isalnum())

t = 'ΑβΓδ'
print(t.isalnum())

t = '20E1'
print(t.isalnum())

t = 'Six lambdas'

print(t.isalnum())

t = 'ΑβΓδ'

print(t.isalnum())

t = '20E1'

print(t.isalnum())

False (the cause of the first result is a space – it’s neither a digit nor a letter.)
True
True

isalpha() – does the string consist only of letters?

# Example 1: Demonstrating the isapha() method:
print("Moooo".isalpha())
print('Mu40'.isalpha())

# Example 1: Demonstrating the isapha() method:

print("Moooo".isalpha())

print('Mu40'.isalpha())

True
False

isdigit() – looks at digits only – anything else produces False as the result.

# Example 2: Demonstrating the isdigit() method:
print('2018'.isdigit())
print("Year2019".isdigit())

# Example 2: Demonstrating the isdigit() method:

print('2018'.isdigit())

print("Year2019".isdigit())

True
False

islower() – does the string consists only of lower-case letters?

# Demonstrating the islower() method:
print("Moooo".islower())
print('moooo'.islower())

# Demonstrating the islower() method:

print("Moooo".islower())

print('moooo'.islower())

False
True

isupper() – does the string consists only of upper-case letters?

# Demonstrating the isupper() method:
print("Moooo".isupper())
print('moooo'.isupper())
print('MOOOO'.isupper())

# Demonstrating the isupper() method:

print("Moooo".isupper())

print('moooo'.isupper())

print('MOOOO'.isupper())

False
False
True

isspace() – does the string consists only of white spaces?

# Example 2: Demonstrating the isspace() method:
print(' \n '.isspace())
print(" ".isspace())
print("mooo mooo mooo".isspace())

# Example 2: Demonstrating the isspace() method:

print(' \n '.isspace())

print(" ".isspace())

print("mooo mooo mooo".isspace())

True
True
False

Exercise 1

What is the length of the following string assuming there is no whitespaces between the quotes?

"""
"""

"""

1

Exercise 2

What is the expected output of the following code?

s = 'yesteryears'
the_list = list(s)
print(the_list[3:6])

s = 'yesteryears'

the_list = list(s)

print(the_list[3:6])

['t', 'e', 'r']

Exercise 3

What is the expected output of the following code?

for ch in "abc":
  print(chr(ord(ch) + 1), end='')

1 2	for ch in "abc": print(chr(ord(ch) + 1), end='')

bcd

Exercise 4

What is the expected output of the following code?

for ch in "abc123XYX":
  if ch.isupper():
    print(ch.lower(), end='')
  elif ch.islower():
    print(ch.upper(), end='')
  else:
    print(ch, end='')

for ch in "abc123XYX":

if ch.isupper():

print(ch.lower(), end='')

elif ch.islower():

print(ch.upper(), end='')

else:

print(ch, end='')

ABC123xyx

Exercise 5

What is the expected output of the following code?

s1 = 'Where are the snows of yesteryear?'
s2 = s1.split()
print(s2[-2])

s1 = 'Where are the snows of yesteryear?'

s2 = s1.split()

print(s2[-2])

of

Exercise 6

What is the expected output of the following code?

the_list = ['Where', 'are', 'the', 'snows?']
s = '*'.join(the_list)
print(s)

the_list = ['Where', 'are', 'the', 'snows?']

s = '*'.join(the_list)

print(s)

Where*are*the*snows?

Exercise 7

What is the expected output of the following code?

s = 'It is either easy or impossible'
s = s.replace('easy', 'hard').replace('im', '')
print(s)

s = 'It is either easy or impossible'

s = s.replace('easy', 'hard').replace('im', '')

print(s)

It is either hard or possible

Slicing

The syntax is:

a[start:stop]  <span class="hljs-comment"># items start through stop-1</span>
a[start:]      <span class="hljs-comment"># items start through the rest of the array</span>
a[:stop]       <span class="hljs-comment"># items from the beginning through stop-1</span>
a[:]           <span class="hljs-comment"># a copy of the whole array</span>

a[start:stop] # items start through stop-1

a[start:] # items start through the rest of the array

a[:stop] # items from the beginning through stop-1

a[:] # a copy of the whole array

There is also the step value, which can be used with any of the above:

a[start:stop:step] <span class="hljs-comment"># start through not past stop, by step</span>

1 2	a[start:stop:step] <span class="hljs-comment"># start through not past stop, by step</span>

The key point to remember is that the :stop value represents the first value that is not in the selected slice. So, the difference between stop and start is the number of elements selected (if step is 1, the default).

The other feature is that start or stop may be a negative number, which means it counts from the end of the array instead of the beginning. So:

a[-<span class="hljs-number">1</span>]    <span class="hljs-comment"># last item in the array</span>
a[-<span class="hljs-number">2</span>:]   <span class="hljs-comment"># last two items in the array</span>
a[:-<span class="hljs-number">2</span>]   <span class="hljs-comment"># everything except the last two items</span>

a[-1] # last item in the array

a[-2:] # last two items in the array

a[:-2] # everything except the last two items

Similarly, step may be a negative number:

a[::-<span class="hljs-number">1</span>]    <span class="hljs-comment"># all items in the array, reversed</span>
a[<span class="hljs-number">1</span>::-<span class="hljs-number">1</span>]   <span class="hljs-comment"># the first two items, reversed</span>
a[:-<span class="hljs-number">3</span>:-<span class="hljs-number">1</span>]  <span class="hljs-comment"># the last two items, reversed</span>
a[-<span class="hljs-number">3</span>::-<span class="hljs-number">1</span>]  <span class="hljs-comment"># everything except the last two items, reversed</span>

a[::-1] # all items in the array, reversed

a[1::-1] # the first two items, reversed

a[:-3:-1] # the last two items, reversed

a[-3::-1] # everything except the last two items, reversed

Python is kind to the programmer if there are fewer items than you ask for. For example, if you ask for a[:-2] and a only contains one element, you get an empty list instead of an error. Sometimes you would prefer the error, so you have to be aware that this may happen.

a = [0, 1, 2, 3, 4, 5, 6, 7, 8, 9]
print(a[::-3]) #  [9, 6, 3, 0]

1 2	a = [0, 1, 2, 3, 4, 5, 6, 7, 8, 9] print(a[::-3]) # [9, 6, 3, 0]

Comparing strings

1. Strings can be compared to strings using general comparison operators, but comparing them to numbers gives no reasonable result, because no string can be equal to any number. Comparing strings against numbers is generally a bad idea. For example:

- string == number is always False;
- string != number is always True;
- string >= number always raises an TypeError exception.

'alpha' < 'alphabet'

1	'alpha' < 'alphabet'

True

String comparison is always case-sensitive (upper-case letters are taken as lesser than lower-case).

'beta' > 'Beta'

1	'beta' > 'Beta'

True

Let’s check it:

'10' == 10
'10' != 10
'10' == 1
'10' != 1
'10' > 10

'10' == 10

'10' != 10

'10' == 1

'10' != 1

'10' > 10

The results in this case are:
False
True
False
True
TypeError exception

2. Sorting lists of strings can be done by:

- a function named sorted(), creating a new, sorted list;

# Demonstrating the sorted() function:
first_greek = ['omega', 'alpha', 'pi', 'gamma']
first_greek_2 = sorted(first_greek)

print(first_greek)
print(first_greek_2)

# Demonstrating the sorted() function:

first_greek = ['omega', 'alpha', 'pi', 'gamma']

first_greek_2 = sorted(first_greek)

print(first_greek)

print(first_greek_2)

['omega', 'alpha', 'pi', 'gamma']

['alpha', 'gamma', 'omega', 'pi']

- a method named sort(), which sorts the list in place

# Demonstrating the sort() method:
second_greek = ['omega', 'alpha', 'pi', 'gamma']
print(second_greek)

second_greek.sort()
print(second_greek)

# Demonstrating the sort() method:

second_greek = ['omega', 'alpha', 'pi', 'gamma']

print(second_greek)

second_greek.sort()

print(second_greek)

['omega', 'alpha', 'pi', 'gamma']

['alpha', 'gamma', 'omega', 'pi']

3. A number can be converted to a string using the str() function.

itg = 13
flt = 1.3
si = str(itg)
sf = str(flt)

print(si + ' ' + sf)

itg = 13

flt = 1.3

si = str(itg)

sf = str(flt)

print(si + ' ' + sf)

The code outputs:
13 1.3

4. A string can be converted to a number (although not every string) using either the int() or float() function. The conversion fails if a string doesn’t contain a valid number image (an exception is raised then).

si = '13'
sf = '1.3'
itg = int(si)
flt = float(sf)

print(itg + flt)

si = '13'

sf = '1.3'

itg = int(si)

flt = float(sf)

print(itg + flt)

This is what you’ll see in the console:
14.3

Exercise 1

Which of the following lines describe a true condition?
'smith' > 'Smith'
'Smiths' < 'Smith'
'Smith' > '1000'
'11' < '8'

1, 3 and 4

Exercise 2

What is the expected output of the following code?

s1 = 'Where are the snows of yesteryear?'
s2 = s1.split()
s3 = sorted(s2)
print(s3[1])
print(s3)

s1 = 'Where are the snows of yesteryear?'

s2 = s1.split()

s3 = sorted(s2)

print(s3[1])

print(s3)

are

['Where', 'are', 'of', 'snows', 'the', 'yesteryear?']

Exercise 3

What is the expected result of the following code?

s1 = '12.8'
i = int(s1)
s2 = str(i)
f = float(s2)
print(s1 == s2)

s1 = '12.8'

i = int(s1)

s2 = str(i)

f = float(s2)

print(s1 == s2)

The code raises a ValueError exception because you can’t int('12.8')

Exercise 4

What will be printed to the monitor ?

w = 'Luke'
x = 'luke'
y = '10'
z = '5'
print(w < x and y < z)
print(x > y and w > z)

w = 'Luke'

x = 'luke'

y = '10'

z = '5'

print(w < x and y < z)

print(x > y and w > z)

This question is all about string comparison.

As a reminder :

-> when comparing strings, Python compares code point values, character by character.

-> string comparison is case-sensitive : upper-case letters are taken as lesser than lower-case

-> when comparing two strings of different lengths and the shorter one is identical to the longer one’s beginning, the longer string is considered greater.

-> when comparing a string to a number (integer or float), the only comparisons you can perform are with the operators == and != operators – other comparison operators (>, <) will raise an exception.

-> code points of number characters (0..9) are less than code points of upper-case characters (A..Z)

-> code points of upper-case characters (A..Z) are less than code points of lower-case characters (a..z)

Note : function ord() returns the code point of a given character.

So, in the above question :

'Luke' < 'luke' is True ( upper-case 'L' has a smaller code point than lower-case 'l' )

'10' < '5' is True ('1' has a smaller code point than '5' )

So : print('Luke' < 'luke' and '10' < '5') will return True

'luke' > '10' is True ( lower-case 'l' has a greater code point than '1' )

'Luke' > '5' is True (upper-case 'L' has a greater code point than '5' )

So : print('luke' > '10' and 'Luke' > '5') will return True

Exercise 5

What is the output of the following code snippet?

print(str(2/3)[-1])

6

Explanation:

2/3 is a valid operation and evaluates to 0.66666666666666666 (a float).

The str() function will convert the float to a string.

str(2/3)[-1] is a slicing operation on the string and will return the last character of the string which is 6 .

Ex. 6

What is the expected output of the following snippet ?

    x = 'Yoda 7'
    if x.isalpha():
        print(1)
    elif x.isdigit():
        print(2)
    elif x.isalnum():
        print(3)
    else:
        print(4)

x = 'Yoda 7'

if x.isalpha():

print(1)

elif x.isdigit():

print(2)

elif x.isalnum():

print(3)

else:

print(4)

4

Explanation:

isalpha() checks if the string contains only alphabetical characters (letters), and returns True or False according to the result.

isdigit() checks if the string contains only digits, and returns True or False according to the result.

isalnum() checks if the string contains only digits or alphabetical characters (letters), and returns True or False according to the result.

Note that a space is neither a digit nor a letter.

So, the above code returns 4.

Ex. 7

What is the expected output of the following code snippet ?

    my_list = ['apple', 'koala', '1234Bz', 'Polo']
    my_list.sort(key = lambda x: x[::-1])
    print(my_list[0])

my_list = ['apple', 'koala', '1234Bz', 'Polo']

my_list.sort(key = lambda x: x[::-1])

print(my_list[0])

Explanation:

sort() is a method of the list class. It is similar to sorted() but with a few differences :

– sorted() returns a new sorted list, while sort() sorts the list in place

– sorted() can be used with an iterable (like a string for example), while sort() can only be used with lists.

Both sort() and sorted() can be used with one of these optional keyword arguments:

– reverse : if set to True, sorting is done in a descending order – if set to False, sorting is done in ascending order.

– key : this argument is a function which will be used on each value in the list being sorted to determine the resulting order.

In the above function, they key argument is used in the sort() method, with key = lambda x: x[::-1] -> this is a lambda function that will be applied on each element of my_list – this lambda function will reverse the order of the characters for each of the strings in my_list . For example : 'apple' will become 'elppa' ; 'koala' will become 'alaok', etc.. The sort() method will then perform the sorting operation based on the first letter of these modified strings ('elppa', 'alaok', etc…) but the original strings are still returned (they are not actually modified by the function identified by key – the function in key is only used to figure out what to sort).

Based on this, 'alaok' would be the first string (because it starts with character ‘a’ which has the smallest Unicode value among lowercase letters). The second one will be 'elppa'. Third one is 'oloP' and last one is 'zB4321'.

And so, my_list.sort(key = lambda x: x[::-1]) will return :

['koala', 'apple', 'Polo', '1234Bz'] (remember that the list is not modified by the function in key, it is just used for ordering).

Ex. 8

Which statement is true about the two objects below ? (Pick two)

str1='Hello'
str2='Hello'

1 2	str1='Hello' str2='Hello'

A. id(str1) == id(str2)

B. str1 is not str2

C. str1[::1] == str2

D. id(str1) != id(str2)

    str1='Hello'
    str2='Hello'
     
    print(id(str1))                # 140473762198000
    print(id(str2))                # 140473762198000
    print(id(str1) == id(str2))    # True
    print(id(str1) != id(str2))    # False
    print(str1[::1])               # Hello
    print(str1[::1] == str2)       # True
    print(str1 is str2)            # True
    print(str1 is not str2)        # False

str1='Hello'

str2='Hello'

print(id(str1)) # 140473762198000

print(id(str2)) # 140473762198000

print(id(str1) == id(str2)) # True

print(id(str1) != id(str2)) # False

print(str1[::1]) # Hello

print(str1[::1] == str2) # True

print(str1 is str2) # True

print(str1 is not str2) # False

Ex. 9

What is the expected output of the following code, assuming there is no whitespace after the 3rd quote of the first line of code?

    my_str = '''
    #'''
     
    print(len(my_str))

my_str = '''

#'''

print(len(my_str))

A. 2

B. 7

C. 8

D. 4

Explanation:

''' allows to delimit a multi-line string.

my_str is a 2-lines string : the first line only includes the special character for a new line : \n – it counts as one character. The 2nd line only includes the # character (which is not considered as a comment in this case).

So, the total length of the string is : 2.

Ex.10

Knowing that ord('a') has a value of 97, which code below will print bee to the monitor ?

A. print(chr(98), chr(101), chr(101), sep=',')

B. print(''.join([str(ord('b')), str(ord('e')), str(ord('e'))]))

C. print(chr(98) + 2*chr(101))

D. print(chr(ord('a')+1), chr(ord('a') + 4), chr(ord('a') + 4))

The Caesar Cipher: encrypting a message

This cipher was (probably) invented and used by Gaius Julius Caesar and his troops during the Gallic Wars. The idea is rather simple – every letter of the message is replaced by its nearest consequent (A becomes B, B becomes C, and so on). The only exception is Z, which becomes A.

# Caesar cipher.
text = input("Enter your message: ")
cipher = ''
for char in text:
    if not char.isalpha():
        continue
    char = char.upper()
    code = ord(char) + 1
    if code > ord('Z'):
        code = ord('A')
    cipher += chr(code)

print(cipher)

# Caesar cipher.

text = input("Enter your message: ")

cipher = ''

for char in text:

if not char.isalpha():

continue

char = char.upper()

code = ord(char) + 1

if code > ord('Z'):

code = ord('A')

cipher += chr(code)

print(cipher)

The Caesar Cipher: decrypting a message

# Caesar cipher - decrypting a message.
cipher = input('Enter your cryptogram: ')
text = ''
for char in cipher:
    if not char.isalpha():
        continue
    char = char.upper()
    code = ord(char) - 1
    if code < ord('A'):
        code = ord('Z')
    text += chr(code)

print(text)

# Caesar cipher - decrypting a message.

cipher = input('Enter your cryptogram: ')

text = ''

for char in cipher:

if not char.isalpha():

continue

char = char.upper()

code = ord(char) - 1

if code < ord('A'):

code = ord('Z')

text += chr(code)

print(text)

The Numbers Processor

The third program shows a simple method allowing you to input a line filled with numbers, and to process them easily. The processing will be extremely easy – we want the numbers to be summed.

# Numbers Processor.

line = input("Enter a line of numbers - separate them with spaces: ")
strings = line.split()
total = 0
try:
    for substr in strings:
        total += float(substr)
    print("The total is:", total)
except:
    print(substr, "is not a number.")

# Numbers Processor.

line = input("Enter a line of numbers - separate them with spaces: ")

strings = line.split()

total = 0

try:

for substr in strings:

total += float(substr)

print("The total is:", total)

except:

print(substr, "is not a number.")

Enter a line of numbers - separate them with spaces: 12 4

The total is: 16.0

The IBAN Validator

The fourth program implements (in a slightly simplified form) an algorithm used by European banks to specify account numbers. The standard named IBAN (International Bank Account Number) provides a simple and fairly reliable method for validating account numbers against simple typos that can occur during rewriting of the number.

The standard says that validation requires the following steps (according to Wikipedia):

(step 1) Check that the total IBAN length is correct as per the country (this program won’t do that, but you can modify the code to meet this requirement if you wish; note: you have to teach the code all the lengths used in Europe)
(step 2) Move the four initial characters to the end of the string (i.e., the country code and the check digits)
(step 3) Replace each letter in the string with two digits, thereby expanding the string, where A = 10, B = 11 … Z = 35;
(step 4) Interpret the string as a decimal integer and compute the remainder of that number by modulo-dividing it by 97; If the remainder is 1, the check digit test is passed and the IBAN might be valid.

# IBAN Validator.

iban = input("Enter IBAN, please: ")
iban = iban.replace(' ','')

if not iban.isalnum():
    print("You have entered invalid characters.")
elif len(iban) < 15:
    print("IBAN entered is too short.")
elif len(iban) > 31:
    print("IBAN entered is too long.")
else:
    iban = (iban[4:] + iban[0:4]).upper()
    iban2 = ''
    for ch in iban:
        if ch.isdigit():
            iban2 += ch
        else:
            iban2 += str(10 + ord(ch) - ord('A'))
    iban = int(iban2)
    if iban % 97 == 1:
        print("IBAN entered is valid.")
    else:
        print("IBAN entered is invalid.")

# IBAN Validator.

iban = input("Enter IBAN, please: ")

iban = iban.replace(' ','')

if not iban.isalnum():

print("You have entered invalid characters.")

elif len(iban) < 15:

print("IBAN entered is too short.")

elif len(iban) > 31:

print("IBAN entered is too long.")

else:

iban = (iban[4:] + iban[0:4]).upper()

iban2 = ''

for ch in iban:

if ch.isdigit():

iban2 += ch

else:

iban2 += str(10 + ord(ch) - ord('A'))

iban = int(iban2)

if iban % 97 == 1:

print("IBAN entered is valid.")

else:

print("IBAN entered is invalid.")