Lesson 5. Strings
A string can be read from the standard input using the function
or defined in single or double quotes. Two strings can be concatenated, and we
can also repeat a string n times multiplying it by integer:
print('>_< ' * 5) # >_< >_< >_< >_< >_<
A string in Python is a sequence of characters. The function
returns how many characters there are in a string:
print(len('abcdefghijklmnopqrstuvwxyz')) # 26
Every object in Python can be converted to string using the function
So we can convert numbers to strings:
s = str(2 ** 100) print(s) # 1267650600228229401496703205376 print(len(s)) # 31
2.1. Single character
A slice gives from the given string one character or some fragment: substring or subsequence.
There are three forms of slices. The simplest form of the slice: a single character
ith character of the string. We count characters starting
from 0. That is, if
Note that in Python there is no separate type for characters of the string.
also has the type
str, just as the source string.
S[i] is called an index.
If you specify a negative index, then it is counted
from the end, starting with the number
-1. That is,
Let's summarize it in the table:
If the index in the slice
S[i] is greater than or equal to
or less than
-len(S), the following error is caused
IndexError: string index out of range.
Slice with two parameters
returns the substring of length
b - a,
starting with the character at index
lasting until the character at index b, not including the last one.
S[1:4] == 'ell', and you can get the same substring using
You can mix positive and negative indexes in the same slice, for example,
a substring without the first and the last character of the string (the slice begins with the character with index 1 and
ends with an index of -1, not including it).
Slices with two parameters never cause
IndexError. For example, for
S == 'Hello'
returns the string
'ello', and the result is the same even
if the second index is very large, like
If you omit the second parameter (but preserve the colon),
then the slice goes to the end of string. For example, to remove
the first character from the string (its index is 0)
take the slice
if you omit the first parameter, then Python takes the slice from the beginning of the string.
That is, to remove the last character from the string, you can use slice
S[:-1]. The slice
S[:] matches the string
Any slice of a string creates a new string and never modifies the original one. In Python strings are immutable, i.e they can not be changed as the objects. You can only assign the variable to the new string, but the old one stays in memory.
In fact in Python there is no variables. There are only the names that are associated with any objects. You can first associate a name with one object, and then — with another. Can several names be associated with one and the same object.
Let's demonstrate that:
s = 'Hello' t = s # s and t point to the same string t = s[2:4] # now t points to the new string 'll' print(s) # prints 'Hello' as s is not changed print(t) # prints 'll'
If you specify a slice with three parameters
the third parameter specifies the step, same as for function
range(). In this case only the characters with the following index are taken:
a + d
a + 2 * d
If the third parameter equals to 2, the slice takes
every second character, and if the step of the slice equals to
-1, the characters go in reverse order.
For example, you can reverse a string like this:
Let's see the examples:
s = 'abcdefg' print(s) print(s[-1]) print(s[1:3]) print(s[1:-1]) print(s[:3]) print(s[2:]) print(s[:-1]) print(s[::2]) print(s[1::2]) print(s[::-1])
s = 'abcdefghijklm' print(s[0:10:2]) for i in range(0, 10, 2): print(i, s[i])
A method is a function that is bound to the object. When the method is called,
the method is applied to the object and does some computations related to it.
Methods are invoked as
For example, in
s.find("e") the string method
find() is applied to
s with one argument
find() searches a substring, passed as an argument,
inside the string on which it's called.
The function returns the index of the first occurrence of the substring.
If the substring is not found, the method returns -1.
s = 'Hello' print(s.find('e')) # 1 print(s.find('ll')) # 2 print(s.find('L')) # -1
Similarly, the method
rfind() returns the index of the last occurrence
of the substring.
s = 'abracadabra' print(s.find('b')) # 1 print(s.rfind('b')) # 8
If you call
find() with three arguments
s.find(substring, left, right), the search is performed
inside the slice
s[left:right]. If you specify only two arguments, like
s.find(substring, left), the search is performed
in the slice
s[left:], that is, starting with the character at index
left to the end of the string. Method
s.find(substring, left, right)
returns the absolute index, relatively to the whole string
s, and not to the slice.
s = 'my name is bond, james bond, okay?' print(s.find('bond')) # 11 print(s.find('bond', 12)) # 23
3.2. replace() method
replace() replaces all occurrences of a given substring with another one. Syntax:
s.replace(old, new) takes the string
S and replaces
all occurrences of substring
old with the substring
print('a bar is a bar, essentially'.replace('bar', 'pub')) # 'a pub is a pub, essentially'
One can pass the third argument
count, like this:
s.replace(old, new, count).
replace() to replace only first
count occurrences and then stop.
print('a bar is a bar, essentially'.replace('bar', 'pub', 1)) # 'a pub is a bar, essentially'
3.3. count() method
This method counts the number of occurrences of one string within another string. The simplest
form is this one:
s.count(substring). Only non-overlapping occurrences are taken into account:
print('Abracadabra'.count('a')) # 4 print(('aaaaaaaaaa').count('aa')) # 5
If you specify three parameters
the count is performed within the slice