Accessing attributes on literals work on all types, but not `int`; why?

I have read that everything in python is an object, and as such I started to experiment with different types and invoking __str__ on them — at first I was feeling really excited, but then I got confused.

>>> "hello world".__str__()
'hello world'
>>> [].__str__()
'[]'
>>> 3.14.__str__()
'3.14'
>>> 3..__str__()
'3.0'
>>> 123.__str__()
  File "<stdin>", line 1
    123.__str__()
              ^
SyntaxError: invalid syntax

• Why does something.str() work for "everything" besides int?
• Is 123 not an object of type int?

You need parens:

(4).__str__()

The problem is the lexer thinks "4." is going to be a floating-point number.

Also, this works:

x = 4
x.__str__()

So you think you can  dance  floating-point?

123 is just as much of an object as 3.14, the "problem" lies within the grammar rules of the language; the parser thinks we are about to define a float — not an int with a trailing method call.

We will get the expected behavior if we wrap the number in parenthesis, as in the below.

>>> (123).str()
'123'

Or if we simply add some whitespace after 123:

>>> 123 .str()
'123'

The reason it does not work for 123.__str__() is that the dot following the 123 is interpreted as the decimal-point of some partially declared floating-point.

>>> 123.str()
File "", line 1
123.str()
^
SyntaxError: invalid syntax

The parser tries to interpret __str__() as a sequence of digits, but obviously fails — and we get a SyntaxError basically saying that the parser stumbled upon something that it did not expect.

---

Elaboration

When looking at 123.__str__() the python parser could use either 3 characters and interpret these 3 characters as an integer, or it could use 4 characters and interpret these as the start of a floating-point.

123.__str__()
^^^ - int

123.__str__()
^^^^- start of floating-point

Just as a little child would like as much cake as possible on their plate, the parser is greedy and would like to swallow as much as it can all at once — even if this isn't always the best of ideas —as such the latter ("better") alternative is chosen.

When it later realizes that __str__() can in no way be interpreted as the decimals of a floating-point it is already too late; SyntaxError.

> ^Note > > 123 .str() # works fine > > In the above snippet, 123  (note the space) must be interpreted as an integer since no number can contain spaces. This means that it is semantically equivalent to (123).__str__().

> ^Note > > 123..str() # works fine > The above also works because a number can contain at most one decimal-point, meaning that it is equivalent to (123.).__str__().

---

For the language-lawyers

This section contains the lexical definition of the relevant literals.

_{Lexical analysis - 2.4.5 Floating point literals}

floatnumber   ::=  pointfloat | exponentfloat
pointfloat    ::=  [intpart] fraction | intpart "."
exponentfloat ::=  (intpart | pointfloat) exponent
intpart       ::=  digit+
fraction      ::=  "." digit+
exponent      ::=  ("e" | "E") ["+" | "-"] digit+

_{Lexical analysis - 2.4.4 Integer literals}

integer        ::=  decimalinteger | octinteger | hexinteger | bininteger
decimalinteger ::=  nonzerodigit digit* | "0"+
nonzerodigit   ::=  "1"..."9"
digit          ::=  "0"..."9"
octinteger     ::=  "0" ("o" | "O") octdigit+
hexinteger     ::=  "0" ("x" | "X") hexdigit+
bininteger     ::=  "0" ("b" | "B") bindigit+
octdigit       ::=  "0"..."7"
hexdigit       ::=  digit | "a"..."f" | "A"..."F"
bindigit       ::=  "0" | "1"

Add a space after the 4:

4 .__str__()

Otherwise, the lexer will split this expression into the tokens "4.", "__str__", "(" and ")", i.e. the first token is interpreted as a floating point number. The lexer always tries to build the longest possible token.

actually (to increase unreadability...):

4..hex()

is valid, too. it gives '0x1.0000000000000p+2' -- but then it's a float, of course...