Get or (re)set the last Python error encountered.

py_clear_last_error()

py_last_error(exception)

Arguments

exception: A python exception object. If provided, the provided exception is set as the last exception.

Value

For py_last_error(), NULL if no error has yet been encountered. Otherwise, a named list with entries:

"type": R string, name of the exception class.
"value": R string, formatted exception message.
"traceback": R character vector, the formatted python traceback,
"message": The full formatted raised exception, as it would be printed in Python. Includes the traceback, type, and value.
"r_trace": A data.frame with class rlang_trace and columns:
- call: The R callstack, full_call, summarized for pretty printing.
- full_call: The R callstack. (Output of sys.calls() at the error callsite).
- parent: The parent of each frame in callstack. (Output of sys.parents() at the error callsite).
- Additional columns for internals use: namespace, visible, scope.

And attribute "exception", a 'python.builtin.Exception' object.

The named list has class

"py_error", and has a default print method that is the equivalent of cat(py_last_error()$message).

Examples

if (FALSE) {

# see last python exception with R traceback
reticulate::py_last_error()

# see the full R callstack from the last Python exception
reticulate::py_last_error()$r_trace$full_call

# run python code that might error,
# without modifying the user-visible python exception

safe_len <- function(x) {
  last_err <- py_last_error()
  tryCatch({
    # this might raise a python exception if x has no `__len__` method.
    import_builtins()$len(x)
  }, error = function(e) {
    # py_last_error() was overwritten, is now "no len method for 'object'"
    py_last_error(last_err) # restore previous exception
    -1L
  })
}

safe_len(py_eval("object"))
}