6.5. String Methods¶
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.
Method | Parameters | Description |
---|---|---|
upper | none | Returns a string in all uppercase |
lower | none | Returns a string in all lowercase |
capitalize | none | Returns a string with first character capitalized, the rest lower |
strip | none | Returns a string with the leading and trailing whitespace removed |
lstrip | none | Returns a string with the leading whitespace removed |
rstrip | none | Returns a string with the trailing whitespace removed |
count | item | Returns the number of occurrences of item |
replace | old, new | Replaces all occurrences of old substring with new |
center | width | Returns a string centered in a field of width spaces |
ljust | width | Returns a string left justified in a field of width spaces |
rjust | width | Returns a string right justified in a field of width spaces |
find | item | Returns the leftmost index where the substring item is found, or -1 if not found |
rfind | item | Returns the rightmost index where the substring item is found, or -1 if not found |
index | item | Like find except causes a runtime error if item is not found |
rindex | item | Like rfind except causes a runtime error if item is not found |
format | substitutions | Involved! See String Format Method, below |
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.
Check your understanding
- 0
- There are definitely o and p characters.
- 2
- There are 2 o characters but what about p?
- 3
- Yes, add the number of o characters and the number of p characters.
strings-5-1: What is printed by the following statements?
s = "python rocks"
print(s.count("o") + s.count("p"))
- yyyyy
- Yes, s[1] is y and the index of n is 5, so 5 y characters. It is important to realize that the index method has precedence over the repetition operator. Repetition is done last.
- 55555
- Close. 5 is not repeated, it is the number of times to repeat.
- n
- This expression uses the index of n
- Error, you cannot combine all those things together.
- This is fine, the repetition operator used the result of indexing and the index method.
strings-5-2: What is printed by the following statements?
s = "python rocks"
print(s[1] * s.index("n"))
6.5.1. String Format Method¶
In grade school quizzes a common convention is to use fill-in-the blanks. For instance,
Hello _____!
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}
.
- Nothing - it causes an error
- It is legal format syntax: put the data in place of the braces.
- sum of {} and {} is {}; product: {}. 2 6 8 12
- Put the data into the format string; not after it.
- sum of 2 and 6 is 8; product: 12.
- Yes, correct substitutions!
- sum of {2} and {6} is {8}; product: {12}.
- Close: REPLACE the braces.
strings-5-3: What is printed by the following statements?
x = 2
y = 6
print('sum of {} and {} is {}; product: {}.'.format( x, y, x+y, x*y))
- 2.34567 2.34567 2.34567
- The numbers before the f in the braces give the number of digits to display after the decimal point.
- 2.3 2.34 2.34567
- Close, but round to the number of digits and display the full number of digits specified.
- 2.3 2.35 2.3456700
- Yes, correct number of digits with rounding!
strings-5-4: What is printed by the following statements?
v = 2.34567
print('{:.1f} {:.2f} {:.7f}'.format(v, v, v))