How to Think Like a Computer Scientist: The PreTeXt Interactive Edition

We previously saw that each turtle instance has its own attributes and a number of methods that can be applied to the instance. For example, we wrote tess.right(90) when we wanted the turtle object tess to perform the right method to turn to the right 90 degrees. The “dot notation” is the way we connect the name of an object to the name of a method it can perform.

Strings are also objects. Each string instance has its own attributes and methods. The most important attribute of the string is the collection of characters. There are a wide variety of methods. Try the following program.

In this example, upper is a method that can be invoked on any string object to create a new string in which all the characters are in uppercase. lower works in a similar fashion changing all characters in the string to lowercase. (The original string ss remains unchanged. A new string tt is created.)

In addition to upper and lower, the following table provides a summary of some other useful string methods. There are a few activecode examples that follow so that you can try them out.

You should experiment with these methods so that you understand what they do. Note once again that the methods that return strings do not change the original. You can also consult the Python documentation for strings

¹

https://docs.python.org/3/library/stdtypes.html#string-methods

.

s = "python rocks"
print(s.count("o") + s.count("p"))

s = "python rocks"
print(s[1] * s.index("n"))

In grade school quizzes a common convention is to use fill-in-the blanks. For instance,

and you can fill in the name of the person greeted, and combine given text with a chosen insertion. We use this as an analogy: Python has a similar construction, better called fill-in-the-braces. The string method format, makes substitutions into places in a string enclosed in braces. Run this code:

There are several new ideas here!

The string for the format method has a special form, with braces embedded. Such a string is called a format string. Places where braces are embedded are replaced by the value of an expression taken from the parameter list for the format method. There are many variations on the syntax between the braces. In this case we use the syntax where the first (and only) location in the string with braces has a substitution made from the first (and only) parameter.

In the code above, this new string is assigned to the identifier greeting, and then the string is printed.

The identifier greeting was introduced to break the operations into a clearer sequence of steps. However, since the value of greeting is only referenced once, it can be eliminated with the more concise version:

There can be multiple substitutions, with data of any type. Next we use floats. Try original price $2.50 with a 7% discount:

The parameters are inserted into the braces in order.

If you used the data suggested, this result is not satisfying. Prices should appear with exactly two places beyond the decimal point, but that is not the default way to display floats.

Format strings can give further information inside the braces showing how to specially format data. In particular floats can be shown with a specific number of decimal places. For two decimal places, put :.2f inside the braces for the monetary values:

The 2 in the format modifier can be replaced by another integer to round to that specified number of digits.

This kind of format string depends directly on the order of the parameters to the format method. There are other approaches that we will skip here, explicitly numbering substitutions and taking substitutions from a dictionary.

A technical point: Since braces have special meaning in a format string, there must be a special rule if you want braces to actually be included in the final formatted string. The rule is to double the braces: { { and }}. For example mathematical set notation uses braces. The initial and final doubled braces in the format string below generate literal braces in the formatted string:

a = 5
b = 9
setStr =  'The set is {​{ {},{} }​}.'.format(a, b)
print(setStr)

Unfortunately, at the time of this writing, the ActiveCode format implementation has a bug, printing doubled braces, but standard Python prints {5, 9}.

You can have multiple placeholders indexing the same argument, or perhaps even have extra arguments that are not referenced at all:

x = 2
y = 6
print('sum of {} and {} is {}; product: {}.'.format( x, y, x+y, x*y))

v = 2.34567
print('{:.1f} {:.2f} {:.7f}'.format(v, v, v))