3.8·Get Length with len()
The len() function counts characters in a string:
>>> letters = 'abcdefg'
>>> len(letters)
7
3.9·Split with split()
Some functions are specific to strings. To use a string function, type thename of the string, a dot, the name of the function, and anyargumentsthat the function needs:
string . function ( arguments )
You can use the built-in stringsplit()function tobreak a string into a listof smaller stringsbased on someseparator.
>>> letters = 'abc,def,ghi'
>>> letters.split(',')
['abc', 'def', 'ghi']
In the preceding example, the string was calledlettersand the string function was called split(), with the single separator argument ','.If you don’t specify a separator, split() uses any sequence of white space characters—newlines, spaces, and tabs.
>>> letters = 'a b c d,ef gh,i'
>>> letters.split()
['a', 'b', 'c', 'd,ef', 'gh,i']
3.10·Combine with join()
Thejoin()function is the opposite of split(): it collapses alistof stringsintoa singlestring.
>>> crypto_list = ['a', 'b', 'c']
>>> crypto_string = '-'.join(crypto_list)
>>> crypto_string
'a-b-c'
3.11·Playing with Strings
Python has a large set ofstring functions. Let’s explore how the most common of them work.
>>> poem = '''All that doth flow we cannot liquid name
Or else would fire and water be the same;
But that is liquid which is moist and wet
Fire that property can never get.
Then 'tis not cold that doth the fire put out
But 'tis the wet that makes it die, no doubt.'''
Does it start with the letters All?
>>> poem.startswith('All')
True
Does it end withThat's all, folks!?
>>> poem.endswith('That\'s all, folks!')
False
let’s find theoffsetof the first occurrence of the wordthein the poem:
>>> poem.find('the')
73
And the offset of the lastthe:
>>> poem.rfind('the')
214
How many times does the three-letter sequencetheoccur?
>>> poem.count('the')
3
Are all of the characters in the poemeither letters or numbers?
>>> poem.isalnum()
False
Nope, there were some punctuation characters.
3.12·Case and Alignment
In this section, we’ll look at some more uses of thebuilt-in string functions.
Our test string is the following:
>>> setup = '...a ..duck goes into a bar...'
>>> setup.strip('.')
'a ..duck goes into a bar' # Remove . sequences from both ends:
Becausestrings are immutable, none of these examples actually changes the setup string. Each example justtakes the value of setup, does something to it, andreturns the result as a new string.
>>> setup = 'a duck goes into a bar...'
>>> setup.capitalize()
'A duck goes into a bar...'
>>> setup.title()
'A Duck Goes Into A Bar...'
>>> setup.upper()
'A DUCK GOES INTO A BAR...'
>>> setup.lower()
'a duck goes into a bar...'
>>> setup = 'a duck goes into a Bar...'
>>> setup.swapcase()
'A DUCK GOES INTO A bAR...'
Now, we’ll work with somelayout alignment functions.
>>> setup = 'a duck goes into a Bar...'
>>> setup.center(30)
' a duck goes into a Bar... '
>>> setup = 'a duck goes into a Bar...'
>>> setup.ljust(30)
'a duck goes into a Bar... '
>>> setup.rjust(30)
' a duck goes into a Bar...'
3.13·Substitute with replace()
You usereplace()for simplesubstring substitution. You give it the old substring, the new one, andhow many instances of the old substring to replace. If you omit this final count argument, it replaces all instances.
>>> setup = 'a b c a b c a'
>>> setup.replace('a', 'd', 2)
'd b c d b c a'
Sometimes, you want to ensure that the substring is a whole word, or the beginning of a word, and so on. In those cases, you needregular expressions.
2017.5.27
Chapter 3. Py Filling: Lists, Tuples, Dictionaries, and Sets
In Chapter 2 we started at the bottom with Python’s basic data types:booleans, integers, floats, and strings. If you think of those as atoms, the data structures in this chapter are like molecules.
1.1·Lists and Tuples