[Python-Dev] TypeError messages (original) (raw)

MRAB python at mrabarnett.plus.com
Fri Feb 20 00:57:10 CET 2015

On 2015-02-19 22:50, Serhiy Storchaka wrote:> Different patterns for TypeError messages are used in the stdlib:

  expected X, Y found
  expected X, found Y
  expected X, but Y found
  expected X instance, Y found
  X expected, not Y
  expect X, not Y
  need X, Y found
  X is required, not Y
  Z must be X, not Y
  Z should be X, not Y

and more.

What the pattern is most preferable?

Stylistically, if the first part is in the active voice, then the second part should also be in the active voice:

 expected X, but found Y

The active voice tends to be simpler and easier to parse than the passive voice.

I think that the word "but" adds clarity here.

Strictly speaking, that message is OK only if it's expecting X itself; if, in fact, it's expecting an instance of X, then you should really be saying something along the lines of:

 expected X instance, but found Y instance

or:

 expected instance of X, but found instance of Y

Some messages use the article before X or Y. Should the article be used or omitted?

Messages tend not to be complete sentences anyway, so I think that it would be fitting to omit articles.

Some messages (only in C) truncate actual type name (%.50s, %.80s, %.200s, %.500s). Should type name be truncated at all and for how limit? Type names newer truncated in TypeError messages raised in Python code.

Truncating type names is probably not a good idea.

Some messages enclose actual type name with single quotes ('%s', '%.200s'). Should type name be quoted? It is uncommon if type name contains spaces.

I think that it should be quoted only if it's expecting those characters, e.g. if it's expecting a closing parenthesis, then it should say ')'. If, on the other hand, it's expecting a certain type, then it should give that type unquoted.

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