Data Type - 'str' for Character String

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∟Data Type - 'str' for Character String

This section describes the 'str' data type, which stores a sequence of Unicode characters.

What Is the "str" Data Type? "str" is the string data type in Python. Each "str" object stores a string, or sequence, of Unicode characters.

"str" data type has the following main Features.

1. "str" data objects can be created in several ways:

Using quoted ASCII strings like 'Say "Hi!"', "Newton's laws", etc.
Using the cast function, str(object), like str(3.14), str([3,1,4]), etc.

Here are some examples on how to create "str" objects:

>>> 'Say "Hi!"'
'Say "Hi!"'

>>> "Newton's laws"
"Newton's laws"

# any special or non-ASCII character can be escaped
>>> 'Say\x0aHi!'
'Say\nHi!'

>>> str(b'Say "Hi!"', 'utf-8')
'Say "Hi!"'

2. "str" data type provides an array-style expression to access the character of a given position.

>>> x = 'apple'
>>> type(x)
<class 'str'>

>>> c = x[0]
>>> c
'a'
>>> type(c)
<class 'str'>

3. "str" data type is an immutable data type. Once a "str" data object is created to store a character sequence, this sequence will never change.

>>> x = 'apple'
>>> c = x[0]
>>> c
'a'

>>> x[0] = 'b'
TypeError: 'str' object does not support item assignment

Don't get confused about "str" immutability and "str" variable re-assignment capability. A "str" variable can be re-assigned with different "bytes" objects many times.

# new object is created for 'apple' and assigned to x
>>> x = 'apple'
>>> id(x)
4459205168

# assign the same object to y
>>> y = x
>>> id(y)
4459205168

# new object is created for 'orange' and assigned to x
>>> x = 'orange'
>>> id(x)
4459205488

# the first object for 'apple' is still there in memory
>>> y
'apple'
>>> id(y)
4459205168

4. Because "str" data type is immutable, an object created for a given character sequence is not allowed to change its value. This allow "str" objects to be cached in memory and reused later whenever they are needed again. Reusing objects will reduce execution time and memory consumption.

On my macOS computer, "str" objects are cached and reused in some cases, not in all cases. The code example below shows you that there are 2 "str" objects created for 4 requests of 'apple'.

>>> x = 'apple'
>>> y = 'apple'
>>> (id(x), id(y), id('apple'), id(str(b"apple", 'ascii')))
(4459205168, 4459205168, 4459205168, 4459205680)

5. "str" objects support the following operations:

    Syntax   Operation       Note
----------   -------------   ------------------
     x + y   Concatenation
     x * n   Repetition      n is an "int"
     n * x   Repetition      Same as x * n
    s in x   Look up         s is a sub "str"
s not in x   Look up         Same as !(s in x)

6. "str" objects support the following comparison operations. They are performed by compare the character value (Unicode point) from each position starting from the left side.

Syntax   Operation
------   ---------
x <  y   Less than
x <= y   Less than or equal
x >  y   Greater than
x >= y   Greater than or equal
x == y   Equal
x != y   Not equal

7. Some built-in functions are provided for "str" objects.

len(x) - Returns the number of characters in this character sequence.
min(x) - Returns the smallest character value in this character sequence.
min(x) - Returns the largest character value in this character sequence.

8. Some instance methods are provided for "str" objects.

str.count(sub) - Instance method returns the number of non-overlapping occurrences of the given subsequence of characters.
str.encode(encoding='utf-8') - Instance method returns a "bytes" object by encoding this character sequence.
str.find(sub) - Instance method returns the lowest index in this character sequence where the given subsequence is found. Returns -1, if the given subsequence is not found.
str.format(*args, **kwargs) - Instance method returns a "str" object by replacing position fields and/or named fields identified with {...}.
str.index(sub) - Instance method returns the lowest index in this character sequence where the given subsequence is found. Raises ValueError, if the given subsequence is not found.
str.join(iterable) - Instance method returns a "str" object by joining "str" objects extracted from the given iterable object, using this character sequence as the separator.
str.partition(sub) - Instance method splits this character sequence at the first occurrence of the given subsequence, and returns a "tuple" containing the part before the subsequence, the subsequence itself, and the part after the subsequence.
str.replace(old, new) - Instance method returns Return a copy of this character sequence with all occurrences of the given old subsequence replaced by given new subsequence.
str.split(sep=None) - Instance method splits this character sequence into tokens against the given separator, and returns resulting tokens a "list" of "str" objects. If the separator is not provided, consecutive whitespace sequence is used as the separator.
str.strip(chars) - Instance method returns a copy of this character sequence with specified chars removed from both ends. If no chars specified, whitespaces will be stripped.
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