[New-bugs-announce] [issue45811] Improve error message when source code contains invisible control characters

[Steven D'Aprano]

New submission from Steven D'Aprano <steve+python at pearwood.info>:

Invisible control characters (aside from white space) are not permitted in source code, but the syntax error we get is confusing and lacks information:

>>> s = 'print\x17("Hello")'
>>> eval(s)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "<string>", line 1
    print("Hello")
         ^
SyntaxError: invalid syntax


The caret points to an invisible character. The offending control character is not visible in the traceback, or the source code unless you use a hex editor. Copying and pasting the string from the traceback, or the source code, may remove the control character (depending on the tools you use), making it even harder to track down the problem.

I suggest that the syntax error should state that the problem is an invisible control character, and display it as a standard human-readable code together with its hex code:

SyntaxError: invisible control character ^W (0x17)


Just in case it isn't obvious what the mapping between controls and the human visible string is:

def control(char):
    n = ord(char)
    if 0 <= n <= 0x1F:
        # C0 control codes
        return '^' + chr(ord('@')+n)
    elif n == 0x7F:
        # DEL
        return '^?'
    elif 0x80 <= n <= 0x9F:
        # C1 control codes
        return 'Esc+' + chr(ord('@')+n-0x80)
    else:
        raise ValueError('Not a control character.')


https://en.wikipedia.org/wiki/C0_and_C1_control_codes

----------
components: Interpreter Core
messages: 406379
nosy: steven.daprano
priority: normal
severity: normal
status: open
title: Improve error message when source code contains invisible control characters
type: enhancement
versions: Python 3.11

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Python tracker <report at bugs.python.org>
<https://bugs.python.org/issue45811>
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