Programming in PythonTypes

Python, like most programming languages, has built-in types for handling common data like numbers and text.

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Numbers

As discussed in the previous section, a numerical value can be either an int or a float. We can represent integers exactly, while storing a real number as a float often requires rounding slightly.

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A number typed directly into a Python program is stored as a float or integer according to whether it contains a decimal point, so if you want the value 6 to be stored as a float, you should write it as 6.0.

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Numbers can be compared using the operators ==,>,<,<=,>=,!=.

1 == 2

Strings

Textual data is represented using a sequence of characters called a string. We can create a string object by enclosing the desired sequence of characters in quotation marks: a = "this is a string". Such a quote-enclosed string of characters in a Python program is called a string literal. String literals can also be delimited by triple quotes, which can be useful for multi-line strings and for strings containing quotes.

"""
This is a multiline string.
It can have "quotes", no problem.
"""

"This is an ordinary string. \"Quotes\" require a backslash."

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We can find the number of characters in a string with the len function: len("hello") returns .

We can concatenate two strings with the addition operator (+): "Hello " + "World".

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We can return the first character in a string s using the expression s[0], the second element using s[1], and so on. We can get the substring from the third to the eighth character using s[2:8]. Note that the 9 is one past the index where we want to stop.

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"Hello World"[i:j]

Solution. Omitting i or j has the effect of setting i = 0 or j = len(s).

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String interpolation

We can insert the value of a variable into a string using string interpolation. There are several ways to do this in Python, but perhaps the simplest is to place an f character immediately before the opening quotation mark. A string literal modified in this way is called an f-string, or formatted string literal. Any parts of an f-string between curly braces are evaluated, and their string representations are inserted into the string at that point.

x = 19
print(f"""
The quotient when x is divided by 3
is {x//3}, and the remainder is {x % 3}.
""")

A = 2
print()

Solution. The expression print(f"multiplying by 6.2 yields {6.2*A}") works.

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Booleans

A bool is a special type whose only values are True and False. The fundamental operators that can be used to combine boolean values are and, or, and not.

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1 == 1.0

Solution. Yes, Python does convert types for equality comparison. So 1 == 1.0 returns True.

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def f(a,b,c):
    pass # add code here

def test_f():
    assert f(True, True, True)
    assert f(False, True, False)
    assert not f(False, True, True)
    return "Tests passed!"

test_f()

Solution. Here's an example of a simple way to do it:

def f(a,b,c):
    return a and b or not c

Be wary of comparisons of the form a == True or b == False. These are equivalent to a and not b, respectively, assuming a and b are both bools. The more succinct versions are preferred.

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Exercises

Solution. We store the length of the given string in a variable a and evaluate the given expression as follows:

a = len("The quick brown fox jumped over the lazy dog")
1/(a+2/3)

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