Why does Pytz have different timezone listings for New York and Detroit?

Question

>>> pytz.country_timezones['US']

America/New_York
America/Detroit
America/Kentucky/Louisville
America/Kentucky/Monticello
America/Indiana/Indianapolis
America/Indiana/Vincennes
America/Indiana/Winamac
America/Indiana/Marengo
America/Indiana/Petersburg
America/Indiana/Vevay
America/Chicago
America/Indiana/Tell_City
America/Indiana/Knox
America/Menominee
America/North_Dakota/Center
America/North_Dakota/New_Salem
America/North_Dakota/Beulah
America/Denver
America/Boise
America/Phoenix
America/Los_Angeles
America/Metlakatla
America/Anchorage
America/Juneau
America/Sitka
America/Yakutat
America/Nome
America/Adak
Pacific/Honolulu

From everything I can find online, there is absolutely no difference between New York and Detroit timezones, including DST. And this was just from checking the first two. They've gotta have a reason for having both, right?

edit: expanding on my research, all the timezones listed which are UTC -5 have the same exact DST info, so it seems even more redundant now. I could remove all the below with one for Eastern Standard Time... Is there anything wrong with doing that?

(UTC -5:00) New_York
(UTC -5:00) Detroit
(UTC -5:00) Kentucky/Louisville
(UTC -5:00) Kentucky/Monticello
(UTC -5:00) Indiana/Indianapolis
(UTC -5:00) Indiana/Vincennes
(UTC -5:00) Indiana/Winamac
(UTC -5:00) Indiana/Marengo
(UTC -5:00) Indiana/Petersburg
(UTC -5:00) Indiana/Vevay

Answer 1

Pytz uses the IANA time zone database (also known as the Olson database). As @brenbarn mentions, time zones in the IANA database reflect "any national region where local clocks have all agreed since [January 1st] 1970.", which includes not only time but DST changes as well.

According to the database, "Most of Michigan observed DST from 1973 on, but was a bit late in 1975." So this means that in 1970 Michigan was on EST, while the east coast observed Daylight Savings Time. Additionally DST did not start on 1975 in Michigan until April 27th, a few weeks after the rest of the Eastern time zone.

If you download the most recent files, there should be various files (northamerica, southamerica, africa, etc.) which have human-readable descriptions of what the different time zones are, and explanations of any changes / reasons why they are not in a different time zone.

---

To build on your edit, Indiana represents a special case within the US, due to its historical presence at the boundary line between two time zones. As of November 2014, Indiana is mostly in the Eastern Time Zone (save for a few counties near Chicago and near Evansville, which are in central time), and the entire state observes Daylight Savings Time. But the Central/Eastern boundary has shifted quite a few times since 1970. In fact, I bet a majority of those time zones happened when a number of counties switched zones in 2006 and again in 2007, adopting Daylight Savings Time in the process.

Answer 2

pytz uses the zoneinfo timezone definitions, which apparently define a time zone as "any national region where local clocks have all agreed since 1970." I would suspect that historical changes in time zone or daylight savings behavior have resulted in locations that currently have the same time but didn't always. (For instance, I think Boise and Denver are also equal timewise now, but they also have different entries.)