Insert space/colon for keywords always followed by space/colon

[krassowski]

There is a number of keywords which are always followed by space or colon in Python:

KEYWORDS_FOLLOWED_BY_SPACE = {
    'as',
    'assert'
    'async',
    'await',
    'class',
    'def',
    'del',
    'for',
    'from',
    'global',
    'import',
    'in',
    'is',
    'nonlocal',
    'raise',
    'while',
    'with',
    # conditional
    'elif'
    'if',
    # logical
    'and',
    'or',
    'not',
}
# note: 'yield' and 'return' can be followed by space but can also be standalone

# keywords that are ALWAYS followed by colon
KEYWORDS_FOLLOWED_BY_COLON = {
    'try',
    'finally'
}
# note: 'else' can be used in a ternary expression: x = 1 if y else 2
# note: 'lambda' can be followed by colon or argument: lambda: 1 or lambda x: x + 1

Many editors modify their insert text to automatically add space/colon in such situations. I would propose that python-lsp-server does the same. Example implementation: https://github.com/krassowski/python-language-server/pull/5/files

Opening to get opinions and feedback.

[krassowski]

Three thoughts:

• An alternative is to include space for yield and return as well, as it is likely that the user may be already using a formatter which strips whitespaces from ends of lines.
• For else we could try to detect whether we are in a ternary expression or not using AST.
• There is nothing we can do for lambda, it is always ambiguous

[ccordoba12]

Sounds good to me.