Why is “1.real” a syntax error but “1 .real” valid in Python?

So I saw these two questions on twitter. How is 1.real a syntax error but 1 .real is not?

>>> 1.real
  File "<stdin>", line 1
    1.real
         ^
SyntaxError: invalid syntax
>>> 1 .real
1
>>> 1. real
  File "<stdin>", line 1
    1. real
          ^
SyntaxError: invalid syntax
>>> 1 . real
1
>>> 1..real
1.0
>>> 1 ..real
  File "<stdin>", line 1
    1 ..real
       ^
SyntaxError: invalid syntax
>>> 1.. real
1.0
>>> 1 .. real
  File "<stdin>", line 1
    1 .. real
       ^
SyntaxError: invalid syntax

Answers:

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Method 1

I guess that the . is greedily parsed as part of a number, if possible, making it the float 1., instead of being part of the method call.

Spaces are not allowed around the decimal point, but you can have spaces before and after the . in a method call. If the number is followed by a space, the parse of the number is terminated, so it’s unambiguous.

Let’s look at the different cases and how they are parsed:

>>> 1.real    # parsed as (1.)real  -> missing '.'
>>> 1 .real   # parsed as (1).real  -> okay
>>> 1. real   # parsed as (1.)real  -> missing '.'
>>> 1 . real  # parsed as (1).real  -> okay
>>> 1..real   # parsed as (1.).real -> okay
>>> 1 ..real  # parsed as (1)..real -> one '.' too much
>>> 1.. real  # parsed as (1.).real -> okay
>>> 1 .. real # parsed as (1)..real -> one '.' too much

Method 2

With 1.real Python is looking for a floating-point numeric literal like 1.0 and you can’t have an r in a float. With 1 .real Python has taken 1 as an integer and is doing the attribute lookup on that.

It’s important to note that the floating-point syntax error handling happens before the . attribute lookup.

Method 3

1 .real works because it is the attribute .real called on the integer 1.

1.real does not work, because you imperatively need a space at the end of a float. Otherwise it is a syntax error.

All methods was sourced from stackoverflow.com or stackexchange.com, is licensed under cc by-sa 2.5, cc by-sa 3.0 and cc by-sa 4.0