In Python 3.6, the timespec argument was added to datetime.isoformat(...) and for good reason: depending on whether or not the datetime object has milliseconds, the isoformat() method returns a different structured string.

Pendulum's to_iso8601_string() has the same issue.

Here's how I resolved it:

    iso_8601 = dt.to_iso8601_string()
    if len(iso_8601) > 25:
        # Contains microseconds and milliseconds. Remove.
        iso_8601 = iso_8601[:19] + iso_8601[26:]

    return iso_8601

However, I think this type of workaround (hack, really) should not be needed and the option to return only to a certain precision (hours, minutes, seconds, milliseconds and microseconds) should be supported in pendulum, as it is in datetime.

Note: this issue is similar to #191 but I think things changed since then, given the support of this feature by Python AND the fact that there are real world scenarios where it's needed.