CommonLounge Archive

Python Strings

March 07, 2018

Python has a built-in string class named str with many handy features (there is an older module named string which you should not use). String literals can be enclosed by either double or single quotes, although single quotes are more commonly used. Backslash escapes work the usual way within both single and double quoted literals — e.g. \n \' \". A double quoted string literal can contain single quotes without any fuss (e.g. "I didn't do it") and likewise single quoted string can contain double quotes.

Python strings are immutable which means they cannot be changed after they are created. Since strings can’t be changed, we construct new strings as we go to represent computed values. So for example the expression 'hello' + 'there' takes in the 2 strings 'hello' and 'there' and builds a new string 'hellothere'.

Characters in a string can be accessed using the standard [ ] syntax. Also, Python uses zero-based indexing (like most other programming languages), so if str is 'hello', str[1] is 'e'. If the index is out of bounds for the string, Python raises an error. The Python style (unlike Perl) is to halt if it can’t tell what to do, rather than just make up a default value. The handy slice syntax (below) also works to extract any substring from a string. The len(string) function returns the length of a string. The [ ] syntax and the len() function actually work on any sequence type — strings, lists, etc.. Python tries to make its operations work consistently across different types. Python newbie gotcha: don’t use len as a variable name to avoid blocking out the len() function. The + operator can concatenate two strings.

s = 'hi'
print s[1]          ## i  
print len(s)        ## 2  
print s + ' there'  ## hi there

The str() function converts values to a string form so they can be combined with other strings.

pi = 3.14  
##text = 'The value of pi is ' + pi      ## NO, does not work  
text = 'The value of pi is '  + str(pi)  ## yes

A raw string literal is prefixed by an r and passes all the chars through without special treatment of backslashes, so r'x\nx' evaluates to the length-4 string 'x\nx'.

raw = r'this\t\n and that'  
print raw     ## this\t\n and that

A string literal can span multiple lines, but there must be a backslash \ at the end of each line to escape the newline. String literals inside triple quotes, """ or ''', can span multiple lines of text.

multi = """It was the best of times.
It was the worst of times."""

String Methods

Here are some of the most common string methods. A method is like a function, but it runs “on” an object. If the variable s is a string, then the code s.lower() runs the lower() method on that string object and returns the result (this idea of a method running on an object is one of the basic ideas that make up Object Oriented Programming, OOP). Here are some of the most common string methods:

  • s.lower(), s.upper() — returns the lowercase or uppercase version of the string
  • s.strip() — returns a string with whitespace removed from the start and end
  • s.isalpha() / s.isdigit() / s.isspace() … — tests if all the string chars are in the various character classes
  • s.startswith('other'), s.endswith('other') — tests if the string starts or ends with the given other string
  • s.find('other') — searches for the given other string (not a regular expression) within s, and returns the first index where it begins or -1 if not found
  • s.replace('old', 'new') — returns a string where all occurrences of 'old' have been replaced by 'new'
  • s.split('delim') — returns a list of substrings separated by the given delimiter. The delimiter is not a regular expression, it’s just text. 'aaa,bbb,ccc'.split(',') gives ['aaa', 'bbb', 'ccc']. As a convenient special case s.split() (with no arguments) splits on all whitespace chars.
  • s.join(list) — opposite of split(), joins the elements in the given list together using the string as the delimiter. e.g. '---'.join(['aaa', 'bbb', 'ccc']) gives aaa---bbb---ccc.

A google search for “python str” should lead you to the official string methods — Python 2.7 documentation which lists all the str methods.

Python does not have a separate character type. Instead an expression like s[8] returns a string-length-1 containing the character. With that string-length-1, the operators ==, <=, … all work as you would expect, so mostly you don’t need to know that Python does not have a separate scalar char type.

String Slices

The slice syntax is a handy way to refer to sub-parts of sequences — typically strings and lists. The slice s[start:end] is the elements beginning at start and extending up to but not including end. Suppose we have s = "Hello"

  • s[1:4] is 'ell' — chars starting at index 1 and extending up to but not including index 4
  • s[1:] is 'ello' — omitting either index defaults to the start or end of the string
  • s[:] is 'Hello' — omitting both always gives us a copy of the whole thing (this is the pythonic way to copy a sequence like a string or list)
  • s[1:100] is 'ello' — an index that is too big is truncated down to the string length

The standard zero-based index numbers give easy access to chars near the start of the string. As an alternative, Python uses negative numbers to give easy access to the chars at the end of the string: s[-1] is the last char 'o', s[-2] is 'l' the next-to-last char, and so on. Negative index numbers count back from the end of the string:

  • s[-1] is 'o' — last char (1st from the end)
  • s[-4] is 'e' — 4th from the end
  • s[:-3] is 'He' — going up to but not including the last 3 chars.
  • s[-3:] is 'llo' — starting with the 3rd char from the end and extending to the end of the string.

It is a neat truism of slices that for any index n, s[:n] + s[n:] == s. This works even for n negative or out of bounds. Or put another way s[:n] and s[n:] always partition the string into two string parts, conserving all the characters. As we’ll see in the list section later, slices work with lists too.

String formatting

Python has a % operator to put together a string. It takes a format string on the left which can have %d (for int), %s (for string), %f/%g (for floating point), and the matching values in a tuple on the right (a tuple is made of values separated by commas, typically grouped inside parentheses). Here’s an example to make it clear.

# % operator
text = "%d little pigs come out or I'll %s and %s and %s" % (3, 'huff', 'puff', 'blow down')

Python will throw an error if you pass an incorrect number of values to be formatted. It will also throw an error is you pass in a string when it expects an integer, and so on.

The above line is kind of long — suppose you want to break it into separate lines. You cannot just split the line after the % as you might in other languages, since by default Python treats each line as a separate statement (on the plus side, this is why we don’t need to type semi-colons on each line). To fix this, enclose the whole expression in an outer set of parenthesis — then the expression is allowed to span multiple lines.

# add parens to make the long-line work:
text = ("%d little pigs come out or I'll %s and %s and %s" %
    (3, 'huff', 'puff', 'blow down'))

Content was originally based on, but has been modified since. Licensed under CC BY 3.0. Code samples licensed under the Apache 2.0 License.

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