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On this page: .index(), .rindex(), .find(), .rfind(), .isalpha(), .isalnum(), .isdigit(), .isspace(), .islower(), .isupper(), string formatting f'{}'.
Locating a Substring
There is a group of string methods whose function is to return the location of a certain substring: .index(), .rindex(), .find() and .rfind().
Below, .index() returns the index of the first substring found within the string, starting from the left edge:
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>>> 'green ideas'.index('e')
2
>>> 'green ideas'.index('id')
6
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.rindex() is the mirror opposite; it searches from the right edge.
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>>> 'green ideas'.rindex('e')
8
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But what if the substring is not found? In such an event, .index() and .rindex() produces an error. Therefore, unless you're sure, it's a good idea to use an in substring test as the precondition:
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>>> phrase = 'green ideas'
>>> phrase.index('th')
Traceback (most recent call last):
File "<pyshell#24>", line 1, in <module>
phrase.index('th')
ValueError: substring not found
>>> if 'th' in phrase:
print(phrase.index('th'))
>>>
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That's where .find() and .rfind() differ from .index() and .rindex(): if the substring is not found, they return -1 instead of producing an error. They behave exactly like .index() and .rindex() otherwise.
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>>> 'green ideas'.find('e')
2
>>> 'green ideas'.rfind('e')
8
>>> 'green ideas'.find('th')
-1
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Is Every Character ... ?
Some string methods test a string on whether or not every character in it possesses a certain property. For example, .isalpha() returns True if and only if every character in the string is alphabetic, i.e., in a-z or A-Z:
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>>> 'iPad'.isalpha()
True
>>> 'co-operate'.isalpha()
False
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Below is a list of similar methods and examples. They all return True/False.
| Method |
Is every character ...? |
Example |
| .isalpha() |
alphabetic (A-Z, a-z) |
See above |
.isalnum() |
alpha-numeric (A-Z, a-z, 0-9) |
>>> 'Homework3'.isalnum()
True
>>> 'ice cream'.isalnum()
False
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.isdigit() |
numeric (0-9) |
>>> '2014'.isdigit()
True
>>> '9:30'.isdigit()
False
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.isspace() |
whitespace character:
' ', '\n', '\t', '\r' |
>>> '\n\t \r \n'.isspace()
True
>>> ' A b c 12 '.isspace()
False
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.islower() |
lowercase (a-z) |
>>> 'apple'.islower()
True
>>> 'iPad'.islower()
False
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.isupper() |
uppercase (A-Z) |
>>> 'HELLO'.isupper()
True
>>> 'Hello'.isupper()
False
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String Formatting
Printing out a bunch of mixed data types can be cumbersome. Below, you're looking for the maximum price and the average price. But before concatenating them, you have to make sure to turn a number into a string, plus the average looks messy with a long trail of decimal digits:
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>>> price = [12.50, 199, 5, 3.75, 33, 66]
>>> max(price)
199
>>> sum(price)/len(price)
53.208333333333336
>>> print('Top price: $' + str(max(price)) + " average: $" + str(sum(price)/len(price)))
Top price: $199 average: $53.208333333333336
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Using string formatting with the f'{}' syntax makes things much neater. Inside the formatted string, you can insert a Python expression enclosed in {}. In addition, you can specify additional formatting syntax: {:.2f} means round the number to the 2nd decimal digit.
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>>> print(f'Top price: ${max(price)} average: ${sum(price)/len(price):.2f}')
Top price: $199 average: $53.21
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Similarly, you can pad digits. Below, we want to print out the date 'Jan 3' as 'Jan 03'. This is accomplished by '{:02d}' which pads 0 on the left to the desired width of 2.
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>>> dates = [('Jan',3), ('Jan',15), ('Mar',7), ('Apr',22)]
>>> for (mo,day) in dates:
print(f'{mo} {day:02d}')
Jan 03
Jan 15
Mar 07
Apr 22
>>>
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