Improve syntax error for numbers with leading zero

[stevendaprano]

BPO	33305
Nosy	@mdickinson, @stevendaprano, @serhiy-storchaka
PRs	bpo-33305: Improve SyntaxError for invalid numerical literals. #6517

^{Note: these values reflect the state of the issue at the time it was migrated and might not reflect the current state.}

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GitHub fields:

assignee = None
closed_at = <Date 2018-07-09.12:30:50.485>
created_at = <Date 2018-04-18.01:47:11.863>
labels = ['interpreter-core', 'type-feature', '3.8']
title = 'Improve syntax error for numbers with leading zero'
updated_at = <Date 2018-07-09.12:30:50.484>
user = 'https://github.com/stevendaprano'

bugs.python.org fields:

activity = <Date 2018-07-09.12:30:50.484>
actor = 'serhiy.storchaka'
assignee = 'none'
closed = True
closed_date = <Date 2018-07-09.12:30:50.485>
closer = 'serhiy.storchaka'
components = ['Interpreter Core']
creation = <Date 2018-04-18.01:47:11.863>
creator = 'steven.daprano'
dependencies = []
files = []
hgrepos = []
issue_num = 33305
keywords = ['patch']
message_count = 7.0
messages = ['315431', '315437', '315439', '315442', '315445', '315724', '321312']
nosy_count = 3.0
nosy_names = ['mark.dickinson', 'steven.daprano', 'serhiy.storchaka']
pr_nums = ['6517']
priority = 'normal'
resolution = 'fixed'
stage = 'resolved'
status = 'closed'
superseder = None
type = 'enhancement'
url = 'https://bugs.python.org/issue33305'
versions = ['Python 3.8']

[stevendaprano]

The Python 2.x syntax for octal integers is a syntax error in 3.x, but the error message is very uninformative:

SyntaxError: invalid token

Can this be improved? Perhaps to something like:

invalid token, use 0o prefix for octal integers

(see also bpo-33304)

[mdickinson]

Maybe once Python 2.7 officially reaches EOL, we can remove the syntax error altogether and allow leading zeros on decimal integer literals.

[mdickinson]

For the message:

invalid token, use 0o prefix for octal integers

I'd expect (without having any evidence to back this up) that the majority of people who encounter this error would be those who intended a decimal literal rather than an octal one, in which case an error message that talks about octal might be confusing.

    >>> import datetime
    >>> birthday = datetime.datetime(1912, 06, 23)
      File "<stdin>", line 1
        birthday = datetime.datetime(1912, 06, 23)
                                            ^
    SyntaxError: invalid token

Could we cover both cases in a single message? "leading zeros in decimal integer literals are not permitted; use an 0o prefix for octal integers"

[serhiy-storchaka]

It still can cause when copy octal constants from other languages or from old Python 2 books and articles.

Perhaps it should emit SyntaxWarning if all digits are in range 0-7 and the number is larger than 7.

[serhiy-storchaka]

PR 6517 improves syntax error messages for invalid numerical literals.

>>> 012
  File "<stdin>", line 1
SyntaxError: leading zeros in decimal integer literals are not permitted; use an 0o prefix for octal integers
>>> 0o129
  File "<stdin>", line 1
SyntaxError: invalid digit '9' in octal literal
>>> 0o
  File "<stdin>", line 1
SyntaxError: invalid octal literal
>>> 1_2_
  File "<stdin>", line 1
SyntaxError: invalid decimal literal
>>> 0.1_2_
  File "<stdin>", line 1
SyntaxError: invalid decimal literal
>>> 12e+
  File "<stdin>", line 1
SyntaxError: invalid decimal literal
>>> 12e+1_
  File "<stdin>", line 1
SyntaxError: invalid decimal literal

"SyntaxError: invalid token" was emitted before.

No tests yet. Suggestions about error messages are welcome.

[serhiy-storchaka]

In Python 2.5 0or[] is valid syntax, but it is not valid in newer versions.

What is the good error message for this case?

[serhiy-storchaka]

New changeset cf7303e by Serhiy Storchaka in branch 'master':
bpo-33305: Improve SyntaxError for invalid numerical literals. (GH-6517)
cf7303e