[PYTHON] Relationship between operators and special methods, try calling numeric methods

Numerical operations have been learned at school, so it doesn't feel strange, but you can also use the +, *, and % operators for character strings. I still understand + and *, but I was surprised that % can be used as a format converter.

>>> 'ABC' + '123'
'ABC123'
>>> 'ABC' * 3
'ABCABCABC'
>>> '%#08x' % 123
'0x00007b'
>>> '%d, %x, %o' % (123, 123, 123)
'123, 7b, 173'

Python is an object-oriented scripting language, and strings will have processing methods, so let's take a look at the method list of strings.

>>> dir('')    # ''The part of can be any character string
['__add__', '__class__', '__contains__', '__delattr__', '__doc__', '__eq__', '__format__', '__ge__', '__getattribute__', '__getitem__', '__getnewargs__', '__getslice__', '__gt__', '__hash__', '__init__', '__le__', '__len__', '__lt__', '__mod__', '__mul__', '__ne__', '__new__', '__reduce__', '__reduce_ex__', '__repr__', '__rmod__', '__rmul__', '__setattr__', '__sizeof__', '__str__', '__subclasshook__', '_formatter_field_name_split', '_formatter_parser', 'capitalize', 'center', 'count', 'decode', 'encode', 'endswith', 'expandtabs', 'find', 'format', 'index', 'isalnum', 'isalpha', 'isdigit', 'islower', 'isspace', 'istitle', 'isupper', 'join', 'ljust', 'lower', 'lstrip', 'partition', 'replace', 'rfind', 'rindex', 'rjust', 'rpartition', 'rsplit', 'rstrip', 'split', 'splitlines', 'startswith', 'strip', 'swapcase', 'title', 'translate', 'upper', 'zfill']

The latter half from capitalize is a normal method for strings.

>>> help(''.capitalize)
Help on built-in function capitalize:

capitalize(...)
    S.capitalize() -> string
    
    Return a copy of the string S with only its first character
    capitalized.

>>> 'ABC'.capitalize()
'Abc'

>>> help(''.center)
Help on built-in function center:

center(...)
    S.center(width[, fillchar]) -> string
    
    Return S centered in a string of length width. Padding is
    done using the specified fill character (default is a space)

>>> 'Python'.center(30, '-')
'------------Python------------'

The first half of the __ (two underscores) displayed by dir ('') is the special method, which seems to be used in a fixed way. __add__, __mul__, __mod__ are the methods called by the+,*,% operators.

>>> 'ABC'.__add__('123')
'ABC123'
>>> 'ABC'.__mul__(3)
'ABCABCABC'
>>> '%#x'.__mod__(123)
'0x7b'

Thanks to the string class defining these methods, you can manipulate strings using operators.

So what about this?

>>> 'ABC' + '123' * 3
'ABC123123123'
>>> 'ABC'.__add__('123').__mul__(3)
'ABC123ABC123ABC123'

The result is different between the above formula and the bottom formula. Operators have precedence, and * takes precedence over+, so the Python interpreter interprets the above expression as'ABC' + ('123' * 3) and: The result is different because the method is called to.

>>> 'ABC'.__add__('123'.__mul__(3))
'ABC123123123'

By the way, let's call the method for numerical values as well.

>>> dir(0)  #0 can be another number
['__abs__', '__add__', '__and__', '__class__', '__cmp__', '__coerce__', '__delattr__', '__div__', '__divmod__', '__doc__', '__float__', '__floordiv__', '__format__', '__getattribute__', '__getnewargs__', '__hash__', '__hex__', '__index__', '__init__', '__int__', '__invert__', '__long__', '__lshift__', '__mod__', '__mul__', '__neg__', '__new__', '__nonzero__', '__oct__', '__or__', '__pos__', '__pow__', '__radd__', '__rand__', '__rdiv__', '__rdivmod__', '__reduce__', '__reduce_ex__', '__repr__', '__rfloordiv__', '__rlshift__', '__rmod__', '__rmul__', '__ror__', '__rpow__', '__rrshift__', '__rshift__', '__rsub__', '__rtruediv__', '__rxor__', '__setattr__', '__sizeof__', '__str__', '__sub__', '__subclasshook__', '__truediv__', '__trunc__', '__xor__', 'bit_length', 'conjugate', 'denominator', 'imag', 'numerator', 'real']
>>> 123.__add__(456)
SyntaxError: invalid syntax
>>> 123 .__add__(456)
579
>>> (123).__add__(456)
579

This is because 123.__ add__ (456) is a syntax error, but . is interpreted as a decimal point. You can call the method by leaving a space or enclosing it in parentheses.

By the way, the built-in function len function calls the __len__ method. Personally, I'd like to define another name for the length () method as well.

>>> len('abcdefg')
7
>>> 'abcdefg'.__len__()
7

There are other special methods used for comparison operators, array access, sequential access, reduce (), etc.