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

Rob Cliffe rob.cliffe at btinternet.com
Fri Feb 20 02:02:43 CET 2015

On 19/02/2015 23:57, MRAB wrote:

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 To me that is a purist argument, but in practice expected int, but found tuple is perfectly clear (I have received such messages myself!), and extra verbiage is just clutter. Identifying Z may be helpful however, so my feeling would be to stick to 2 patterns - the one above and Z should be int, not tuple (You may prefer e.g. "got" to "found", or "must" to "should". But ideally the usage should be the same throughout.) Rob Cliffe

> 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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