Functions and Methods on Strings

Strings are a deceptively broad topic. We'll cover only the highlights in this section, and I will lead you to the sources of more information so that you can be familiar with what strings have to offer as to avoid you "re-inventing the wheel".

"Secret Codes"

Everything in computers relate back to binary ('0's and '1's) as the information is stored and processed. Computers don't inherently know what text is, much less English text. A standard needed to be created to map letters and charcters of a keyboard into its numberic binary equivalent.

Let's start with a string, below. The odd upper- and lower-casing is intentional.

In [16]:
my_str = "FoOBar"

Using the ord function, we can get the "ordinal" or ranking number of a character in the ASCII table. The letter 'F' is mapped to the decimal (base 10) number 70, which is equivalent to the binary number (base 2) of 1000110. This binary representing is how the 'F' character is stored in your text file and in memory. We just build a system of mapping representation so humans can type and see the letter 'F' without worrying about typing "1000110". That's the ASCII standard.

In [17]:
ord(my_str[0])
Out[17]:
70

The bin() function is used to turn a base-10 integer (decimal) into a base-2 integer (binary).

In [18]:
bin(ord(my_str[0]))
Out[18]:
'0b1000110'

The paired function to ord() is the chr() function. When given an integer, it returns the character that is mapped to that integer. I can provide a decimal or binary base integer for that (binary integers are series of '1's and '0's prepended with '0b', see below.

In [19]:
chr(70)
Out[19]:
'F'
In [20]:
chr(0b1000110)
Out[20]:
'F'

Let's look at all the characters in the string converted to their ASCII number in decimal and binary.

In [21]:
for c in my_str:
    print(c, ord(c), bin(ord(c)))
F 70 0b1000110
o 111 0b1101111
O 79 0b1001111
B 66 0b1000010
a 97 0b1100001
r 114 0b1110010

Speaking of binary, Futurama is a great TV show. Bender uses a "binary Time Code" to travel through time in Bender's Big Score.

String Methods

Every string you use is, again, an object. Not only do you have the string of characters that make up the string, but the useful methods (functions) included with it to act upon this string.

How do you know what to you? Use online documentation at http://python.org, or the very helpful help() function built right into the interpreter. The help() function can take any literal or identifier (since they all refer to an object) to provide helpful documentation about that object data type.

Note: Well, actually, in Python 3 you can't give a string object for help, you have to provide the class (str) instead. I'm not sure why this is the case, but it's the one outlier I've found.

In [22]:
help(str)
Help on class str in module builtins:

class str(object)
 |  str(object='') -> str
 |  str(bytes_or_buffer[, encoding[, errors]]) -> str
 |  
 |  Create a new string object from the given object. If encoding or
 |  errors is specified, then the object must expose a data buffer
 |  that will be decoded using the given encoding and error handler.
 |  Otherwise, returns the result of object.__str__() (if defined)
 |  or repr(object).
 |  encoding defaults to sys.getdefaultencoding().
 |  errors defaults to 'strict'.
 |  
 |  Methods defined here:
 |  
 |  __add__(self, value, /)
 |      Return self+value.
 |  
 |  __contains__(self, key, /)
 |      Return key in self.
 |  
 |  __eq__(self, value, /)
 |      Return self==value.
 |  
 |  __format__(...)
 |      S.__format__(format_spec) -> str
 |      
 |      Return a formatted version of S as described by format_spec.
 |  
 |  __ge__(self, value, /)
 |      Return self>=value.
 |  
 |  __getattribute__(self, name, /)
 |      Return getattr(self, name).
 |  
 |  __getitem__(self, key, /)
 |      Return self[key].
 |  
 |  __getnewargs__(...)
 |  
 |  __gt__(self, value, /)
 |      Return self>value.
 |  
 |  __hash__(self, /)
 |      Return hash(self).
 |  
 |  __iter__(self, /)
 |      Implement iter(self).
 |  
 |  __le__(self, value, /)
 |      Return self<=value.
 |  
 |  __len__(self, /)
 |      Return len(self).
 |  
 |  __lt__(self, value, /)
 |      Return self<value.
 |  
 |  __mod__(self, value, /)
 |      Return self%value.
 |  
 |  __mul__(self, value, /)
 |      Return self*value.n
 |  
 |  __ne__(self, value, /)
 |      Return self!=value.
 |  
 |  __new__(*args, **kwargs) from builtins.type
 |      Create and return a new object.  See help(type) for accurate signature.
 |  
 |  __repr__(self, /)
 |      Return repr(self).
 |  
 |  __rmod__(self, value, /)
 |      Return value%self.
 |  
 |  __rmul__(self, value, /)
 |      Return self*value.
 |  
 |  __sizeof__(...)
 |      S.__sizeof__() -> size of S in memory, in bytes
 |  
 |  __str__(self, /)
 |      Return str(self).
 |  
 |  capitalize(...)
 |      S.capitalize() -> str
 |      
 |      Return a capitalized version of S, i.e. make the first character
 |      have upper case and the rest lower case.
 |  
 |  casefold(...)
 |      S.casefold() -> str
 |      
 |      Return a version of S suitable for caseless comparisons.
 |  
 |  center(...)
 |      S.center(width[, fillchar]) -> str
 |      
 |      Return S centered in a string of length width. Padding is
 |      done using the specified fill character (default is a space)
 |  
 |  count(...)
 |      S.count(sub[, start[, end]]) -> int
 |      
 |      Return the number of non-overlapping occurrences of substring sub in
 |      string S[start:end].  Optional arguments start and end are
 |      interpreted as in slice notation.
 |  
 |  encode(...)
 |      S.encode(encoding='utf-8', errors='strict') -> bytes
 |      
 |      Encode S using the codec registered for encoding. Default encoding
 |      is 'utf-8'. errors may be given to set a different error
 |      handling scheme. Default is 'strict' meaning that encoding errors raise
 |      a UnicodeEncodeError. Other possible values are 'ignore', 'replace' and
 |      'xmlcharrefreplace' as well as any other name registered with
 |      codecs.register_error that can handle UnicodeEncodeErrors.
 |  
 |  endswith(...)
 |      S.endswith(suffix[, start[, end]]) -> bool
 |      
 |      Return True if S ends with the specified suffix, False otherwise.
 |      With optional start, test S beginning at that position.
 |      With optional end, stop comparing S at that position.
 |      suffix can also be a tuple of strings to try.
 |  
 |  expandtabs(...)
 |      S.expandtabs(tabsize=8) -> str
 |      
 |      Return a copy of S where all tab characters are expanded using spaces.
 |      If tabsize is not given, a tab size of 8 characters is assumed.
 |  
 |  find(...)
 |      S.find(sub[, start[, end]]) -> int
 |      
 |      Return the lowest index in S where substring sub is found,
 |      such that sub is contained within S[start:end].  Optional
 |      arguments start and end are interpreted as in slice notation.
 |      
 |      Return -1 on failure.
 |  
 |  format(...)
 |      S.format(*args, **kwargs) -> str
 |      
 |      Return a formatted version of S, using substitutions from args and kwargs.
 |      The substitutions are identified by braces ('{' and '}').
 |  
 |  format_map(...)
 |      S.format_map(mapping) -> str
 |      
 |      Return a formatted version of S, using substitutions from mapping.
 |      The substitutions are identified by braces ('{' and '}').
 |  
 |  index(...)
 |      S.index(sub[, start[, end]]) -> int
 |      
 |      Return the lowest index in S where substring sub is found, 
 |      such that sub is contained within S[start:end].  Optional
 |      arguments start and end are interpreted as in slice notation.
 |      
 |      Raises ValueError when the substring is not found.
 |  
 |  isalnum(...)
 |      S.isalnum() -> bool
 |      
 |      Return True if all characters in S are alphanumeric
 |      and there is at least one character in S, False otherwise.
 |  
 |  isalpha(...)
 |      S.isalpha() -> bool
 |      
 |      Return True if all characters in S are alphabetic
 |      and there is at least one character in S, False otherwise.
 |  
 |  isdecimal(...)
 |      S.isdecimal() -> bool
 |      
 |      Return True if there are only decimal characters in S,
 |      False otherwise.
 |  
 |  isdigit(...)
 |      S.isdigit() -> bool
 |      
 |      Return True if all characters in S are digits
 |      and there is at least one character in S, False otherwise.
 |  
 |  isidentifier(...)
 |      S.isidentifier() -> bool
 |      
 |      Return True if S is a valid identifier according
 |      to the language definition.
 |      
 |      Use keyword.iskeyword() to test for reserved identifiers
 |      such as "def" and "class".
 |  
 |  islower(...)
 |      S.islower() -> bool
 |      
 |      Return True if all cased characters in S are lowercase and there is
 |      at least one cased character in S, False otherwise.
 |  
 |  isnumeric(...)
 |      S.isnumeric() -> bool
 |      
 |      Return True if there are only numeric characters in S,
 |      False otherwise.
 |  
 |  isprintable(...)
 |      S.isprintable() -> bool
 |      
 |      Return True if all characters in S are considered
 |      printable in repr() or S is empty, False otherwise.
 |  
 |  isspace(...)
 |      S.isspace() -> bool
 |      
 |      Return True if all characters in S are whitespace
 |      and there is at least one character in S, False otherwise.
 |  
 |  istitle(...)
 |      S.istitle() -> bool
 |      
 |      Return True if S is a titlecased string and there is at least one
 |      character in S, i.e. upper- and titlecase characters may only
 |      follow uncased characters and lowercase characters only cased ones.
 |      Return False otherwise.
 |  
 |  isupper(...)
 |      S.isupper() -> bool
 |      
 |      Return True if all cased characters in S are uppercase and there is
 |      at least one cased character in S, False otherwise.
 |  
 |  join(...)
 |      S.join(iterable) -> str
 |      
 |      Return a string which is the concatenation of the strings in the
 |      iterable.  The separator between elements is S.
 |  
 |  ljust(...)
 |      S.ljust(width[, fillchar]) -> str
 |      
 |      Return S left-justified in a Unicode string of length width. Padding is
 |      done using the specified fill character (default is a space).
 |  
 |  lower(...)
 |      S.lower() -> str
 |      
 |      Return a copy of the string S converted to lowercase.
 |  
 |  lstrip(...)
 |      S.lstrip([chars]) -> str
 |      
 |      Return a copy of the string S with leading whitespace removed.
 |      If chars is given and not None, remove characters in chars instead.
 |  
 |  partition(...)
 |      S.partition(sep) -> (head, sep, tail)
 |      
 |      Search for the separator sep in S, and return the part before it,
 |      the separator itself, and the part after it.  If the separator is not
 |      found, return S and two empty strings.
 |  
 |  replace(...)
 |      S.replace(old, new[, count]) -> str
 |      
 |      Return a copy of S with all occurrences of substring
 |      old replaced by new.  If the optional argument count is
 |      given, only the first count occurrences are replaced.
 |  
 |  rfind(...)
 |      S.rfind(sub[, start[, end]]) -> int
 |      
 |      Return the highest index in S where substring sub is found,
 |      such that sub is contained within S[start:end].  Optional
 |      arguments start and end are interpreted as in slice notation.
 |      
 |      Return -1 on failure.
 |  
 |  rindex(...)
 |      S.rindex(sub[, start[, end]]) -> int
 |      
 |      Return the highest index in S where substring sub is found,
 |      such that sub is contained within S[start:end].  Optional
 |      arguments start and end are interpreted as in slice notation.
 |      
 |      Raises ValueError when the substring is not found.
 |  
 |  rjust(...)
 |      S.rjust(width[, fillchar]) -> str
 |      
 |      Return S right-justified in a string of length width. Padding is
 |      done using the specified fill character (default is a space).
 |  
 |  rpartition(...)
 |      S.rpartition(sep) -> (head, sep, tail)
 |      
 |      Search for the separator sep in S, starting at the end of S, and return
 |      the part before it, the separator itself, and the part after it.  If the
 |      separator is not found, return two empty strings and S.
 |  
 |  rsplit(...)
 |      S.rsplit(sep=None, maxsplit=-1) -> list of strings
 |      
 |      Return a list of the words in S, using sep as the
 |      delimiter string, starting at the end of the string and
 |      working to the front.  If maxsplit is given, at most maxsplit
 |      splits are done. If sep is not specified, any whitespace string
 |      is a separator.
 |  
 |  rstrip(...)
 |      S.rstrip([chars]) -> str
 |      
 |      Return a copy of the string S with trailing whitespace removed.
 |      If chars is given and not None, remove characters in chars instead.
 |  
 |  split(...)
 |      S.split(sep=None, maxsplit=-1) -> list of strings
 |      
 |      Return a list of the words in S, using sep as the
 |      delimiter string.  If maxsplit is given, at most maxsplit
 |      splits are done. If sep is not specified or is None, any
 |      whitespace string is a separator and empty strings are
 |      removed from the result.
 |  
 |  splitlines(...)
 |      S.splitlines([keepends]) -> list of strings
 |      
 |      Return a list of the lines in S, breaking at line boundaries.
 |      Line breaks are not included in the resulting list unless keepends
 |      is given and true.
 |  
 |  startswith(...)
 |      S.startswith(prefix[, start[, end]]) -> bool
 |      
 |      Return True if S starts with the specified prefix, False otherwise.
 |      With optional start, test S beginning at that position.
 |      With optional end, stop comparing S at that position.
 |      prefix can also be a tuple of strings to try.
 |  
 |  strip(...)
 |      S.strip([chars]) -> str
 |      
 |      Return a copy of the string S with leading and trailing
 |      whitespace removed.
 |      If chars is given and not None, remove characters in chars instead.
 |  
 |  swapcase(...)
 |      S.swapcase() -> str
 |      
 |      Return a copy of S with uppercase characters converted to lowercase
 |      and vice versa.
 |  
 |  title(...)
 |      S.title() -> str
 |      
 |      Return a titlecased version of S, i.e. words start with title case
 |      characters, all remaining cased characters have lower case.
 |  
 |  translate(...)
 |      S.translate(table) -> str
 |      
 |      Return a copy of the string S in which each character has been mapped
 |      through the given translation table. The table must implement
 |      lookup/indexing via __getitem__, for instance a dictionary or list,
 |      mapping Unicode ordinals to Unicode ordinals, strings, or None. If
 |      this operation raises LookupError, the character is left untouched.
 |      Characters mapped to None are deleted.
 |  
 |  upper(...)
 |      S.upper() -> str
 |      
 |      Return a copy of S converted to uppercase.
 |  
 |  zfill(...)
 |      S.zfill(width) -> str
 |      
 |      Pad a numeric string S with zeros on the left, to fill a field
 |      of the specified width. The string S is never truncated.
 |  
 |  ----------------------------------------------------------------------
 |  Static methods defined here:
 |  
 |  maketrans(x, y=None, z=None, /)
 |      Return a translation table usable for str.translate().
 |      
 |      If there is only one argument, it must be a dictionary mapping Unicode
 |      ordinals (integers) or characters to Unicode ordinals, strings or None.
 |      Character keys will be then converted to ordinals.
 |      If there are two arguments, they must be strings of equal length, and
 |      in the resulting dictionary, each character in x will be mapped to the
 |      character at the same position in y. If there is a third argument, it
 |      must be a string, whose characters will be mapped to None in the result.

As you can see, there are many methods for manipulating strings. Please be aware that manipulation* of a string means creating a new string from an existing one as strings are immutable (unchangeable) after they have been created.

For example, what if you want to capitialize all the characters in the string my_str?

You would use this form: my_str.upper(). This does not modify the string my_str! It creates a new string from that data to make a new string, so you need to assign the new string to a variable or something to make it useful. e.g.:

In [23]:
upper_str = my_str.upper()
print(my_str, upper_str)
FoOBar FOOBAR

.join()

Another immediately useful method is .join(). This string method joins together a list of strings (or, a string of characters) with whatever characters on which you are calling the string you are calling .join() method.

For example, this example really fubars everything, by joining the string

In [24]:
my_str.join("Spam")
Out[24]:
'SFoOBarpFoOBaraFoOBarm'

This example is more practical and inline with what I would use as a programmer. Given a list of strings, ["Steve", "Jobs"], I use the string " " (a single space character) to join together that list of strings into one newly returned string "Steve Jobs".

In [25]:
" ".join(["Steve", "Jobs"])
Out[25]:
'Steve Jobs'
In [26]:
space_chr = " "
space_chr.join(["Steve", "Jobs"])
Out[26]:
'Steve Jobs'
In [27]:
space_chr.join(["James", "Tiberius", "Kirk"])
Out[27]:
'James Tiberius Kirk'
In [28]:
" ".join("SteveJobs")
Out[28]:
'S t e v e J o b s'

.split()

The opposite of .join() is .split(). This takes a string a breaks it up into smaller strings. This function returns a list of strings as a result.

By default, if now parameter is provided to .split(), it breaks strings up anywhere "whitespace" occurs. This is typically the following characters:

  • space
  • tab
  • newline
  • carriage return
  • A couple others…

Usually, you use this default form to break up words separated by spaces (word boundaries).

In [29]:
# We can use the helper module `string` to see what Python defines as 'whitespace'.
import string
string.whitespace
Out[29]:
' \t\n\r\x0b\x0c'
In [30]:
crazy = 'S t e v e J o b s'
crazy.split()
Out[30]:
['S', 't', 'e', 'v', 'e', 'J', 'o', 'b', 's']
In [31]:
name = 'Steve Jobs'
name.split()
Out[31]:
['Steve', 'Jobs']
In [32]:
name = 'Steve_Jobs'
name.split()
Out[32]:
['Steve_Jobs']

However, .split() can be quite flexible with the characters used for breaking up a string. You can provide a string of a single or multiple characters that will break up a string; however note if you said:

name.split("*_|")

You would expect that the string name had the character sequence of "*_| separating characters, eg. Steve*_|Jobs. In other words, when you provide characters for .split() it splits based on the entire sequence, not individual characters in that provided sequence.

In [33]:
name = 'Steve_Jobs'
name.split("_")
Out[33]:
['Steve', 'Jobs']

Please review the full menifest of string methods and be familiar with the general use of them!