utcnow 0.3.8

Timestamps as opinionated RFC 3339 (Date and Time on the Internet) formatted strings

utcnow

Timestamps as RFC 3339 (Date & Time on the Internet) formatted strings with conversion functionality from other timestamp formats or for timestamps on other timezones. Additionally converts timestamps from datetime objects and other common date utilities. Follow modern practices when developing API interfaces.

A library for formatting timestamps as RFC 3339

• One preferred output format strictly following RFC 3339.
• Making it easier to follow common best practices for developers.
• Tranforming timestamps from (or when needed, to) common interfaces such as unix timestamps, datetime objects, google.protobuf.Timestamp messages or plain text.
• Type hinted, battle tested and supporting several versions of Python.

>>> utcnow.rfc3339_timestamp()  # current time
'2022-12-06T12:25:49.738032Z'

>>> utcnow.rfc3339_timestamp("1984-08-01 22:31")
'1984-08-01T22:31:00.000000Z'

Additional examples of simple use-cases

from utcnow import utcnow

utcnow.get()  # utcnow.get does the same thing as utcnow.rfc3339_timestamp
# "2077-03-01T09:33:07.139361Z" | The most common use case – get current server time.
#                               | Always uses UTC timezone in the returned value.
#                               | utcnow.get() is the same as utcnow.rfc3339_timestamp().

from utcnow import utcnow

utcnow.get("2020-02-26 09:10:10+00:00")
# "2020-02-26T09:10:10.000000Z" | Reformats any valid date-time input to a defined standard.
#                               | RFC 3339 compliant: YYYY-MM-DDTHH:mm:ss.ffffffZ

utcnow.get("1997-08-04T02:14:00.53-04:00")
# "1997-08-04T06:14:00.530000Z" | Timezones as UTC for aligned and clean interfaces.
#                               | Uses "Z", Zulu Time, to specify UTC timezone.

utcnow.get("1989-12-13 08:35 UTC")
# "1989-12-13T08:35:00.000000Z" | Converts from different input formats and patterns.
#                               | Any other RFC 3339 compliant input is valid + more.

# 👋 Look further down for additional code examples of other types of input values.

The elevator pitch – purpose for developers – the why

NOTE – OPINIONATED SOFTWARE

utcnow is opinionated about the format of string based timestamps. For example, that timestamps as strings should be stored using the same formatting and preferably using the same length, as well as adhering to the current best practices – which for computer-to-computer comms should be by following "RFC 3339 (Date and Time on the Internet: Timestamps)".

TIMESTAMPS WILL USE UTC

String based timestamps that are meant for logs, API responses and database records shall always be stored with timezone UTC.

---

Someone – somewhere: "Why UTC? It's not even a timezone for our markets."

Devs (and wikipedia): "Coordinated Universal Time or Universal Time Coordinated, UTC for short, is still currently the primary time standard and is not affected by daylight saving time, which is usually not something that servers or software developers would want to work around."

"It's pretty simple – modern internet applications shouldn't use any other timezone in their databases, logs, API:s or other computer to computer interfaces."

---

Good timestamps and UTC – really no wild and crazy opinions. Generailly this lib is just about making it easier to follow common best practices harder to do something wrong – and that's also why utcnow doesn't have any configuration options. The library does what it should do – "shoganai".

RULES FOR RETURNED TIMESTAMPS

The following ruleset are applied to timestamps returned by utcnow when requesting a string based format:

• Timestamps follow RFC 3339 (Date and Time on the Internet: Timestamps): https://tools.ietf.org/html/rfc3339.
• Timestamps are converted to UTC timezone which we'll note in the timestamp with the "Z" syntax instead of the also accepted "+00:00". "Z" stands for UTC+0 or "Zulu time" and refers to the zone description of zero hours.
• Timestamps are expressed as a date-time, including the full date (the "T" between the date and the time is optional in RFC 3339 (but not in ISO 8601) and usually describes the beginning of the time part.
• Timestamps are 27 characters long in the format: "YYYY-MM-DDTHH:mm:ss.ffffffZ". 4 digit year, 2 digit month, 2 digit days. "T", 2 digit hours, 2 digit minutes, 2 digit seconds, 6 fractional second digits (microseconds -> nanoseconds), followed by the timezone identifier for UTC: "Z".

utcnow is defined to return timestamps with 6 fractional second digits, which means timestamps down to the microsecond level. Having a six-digit fraction of a second is currently the most common way that timestamps are shown at this date.

When using a fixed length return value for string based timestamps it'll even make the returned strings comparable to each other.

Where to use this – for what kind of applications or interfaces

Some examples of timestamps where this formatting would be reasonable to use includes, but are not limited to any timestamp that is written to a database / datastore as a string, also when timestamps are used in log output or used within a JSON response for an API such as a REST or GraphQL based API, maybe even using custom DateTime scalars.

If any of this sounds like the use-cases within your domains, try utcnow out – might do the trick.

If your work require a complex mix and match back and forth using different timezones even within internal applications (which may be true for legacy systems or on purely domestic use-cases), then go for arrow. Also iterating: Modern internet applications shouldn't use any other timezone than UTC in app to app / computer to computer interfaces.

Note that this library is built with backend developers in mind and while these date-time formatted timestamps are more readable than unix timestamps, they still usually shouldn't be used within user interfaces, however since these format of timestamps are so common basically any library will be able to convert back and forth into whatever your human users would expect, to improve readability – useful for frontend applications, app user interfaces, etc. Also, using a standard like this, the frontend devs won't banish you for changing formatting of timestamps within API responses across different services.

Supported input values for timestamp conversion

This library aims at going for simplicity by being explicit about the choices allowed to make. utcnow however allows the conversion methods to be called with the following kind of argument values:

• RFC 3339 compliant strings, which at the very least must include the full date, but could omit the time part of a date-time, leaving only the date, or by not including the seconds, microseconds or even laving out the timezone information – utcnow supports all of the use-cases of RFC 3339 inputs and then converts the input into an even more complete RFC 3339 timestamp in UTC timezone.
• The most common format for handling dates and datetimes in Python, the builtin datetime.datetime object values (both timezone aware values, as well as values that aren't timezone aware, as for which we'll assume UTC).
• Also supporting object values from other commonly used libraries, such as arrow.
• As a bonus – Unix time, mainly for convinience (time.time()) (we have many names for the things we love: epoch time, posix time, seconds since epoch, 2038-bug on 32-bit unsigned ints to time-travel back to the first radio-transmission across the atlantic, there will be movies about this ).

Can also be used on the command-line

utcnow can now be used as a cli by installing utcnow-cli or using the cli extras of utcnow.

# install utcnow with extras: cli
pip install utcnow[cli]
# equivalent to installing utcnow-cli
pip install utcnow-cli

code ~$ utcnow
2022-10-17T14:25:04.481821Z

code ~$ utcnow +365d
2023-10-17T14:25:04.481821Z

code ~$ utcnow "2022-02-28 10:10:59.100+02:00" "1984-08-01 15:00"
2022-02-28T08:10:59.100000Z
1984-08-01T15:00:00.000000Z

FULL CLI USAGE

  usage:
    utcnow [values ...]              | default     output in rfc3339 format
    utcnow --unixtime [values ...]   | short: -u   output as unixtime
    utcnow --diff <from> <to>        | short: -d   diff in seconds: from -> to

  help:
    utcnow --help                    | short: -h   display this message
    utcnow --version                 | short: -v   show installed version

A neat side-effect of defaulted string output – comparison as strings

If date and time components are ordered from least precise to most precise, then a useful property is achieved. Assuming that the time zones of the dates and times are the same (e.g., all in UTC), expressed using the same string (e.g., all "Z" or all "+00:00"), and all times have the same number of fractional second digits, then the date and time strings may be sorted as strings and a time-ordered sequence will result. he presence of optional punctuation would violate this characteristic.

Here follows a few examples of the problems with having to work with mismatching timestamps, even though the four example statements all use RFC 3339 compliant values. For example an API is kind enough for users to submit timestamps as long as they're good enough and for where the backend application has to convert inputs to values good for the cause.

Matching two dates of different formats using strings won't go well at all. All of the following four string comparisons would've given an opposite result if compared as actual timestamps instead of as strings, where comparison is just alphabetic.

"2022-08-01 23:51:30.000000Z"          >  "2022-08-01T13:51:30.000000Z"          # False 😵
"2022-08-01 14:00:10"                  >  "2022-08-01T13:51:30.000000Z"          # False 😵
"2022-08-01T14:00:10+01:00"            >  "2022-08-01T13:51:30.000000Z"          # True  😵
"2022-08-01T13:51Z"                    >  "2022-08-01T13:51:30.000000Z"          # True  😵

Using utcnow on the same set of timestamps, which returns a string value for comparison.

from utcnow import utcnow

utcnow("2022-08-01 23:51:30.000000Z")  >  utcnow("2022-08-01T13:51:30.000000Z")  # True  🎉
utcnow("2022-08-01 14:00:10")          >  utcnow("2022-08-01T13:51:30.000000Z")  # True  ✅
utcnow("2022-08-01T14:00:10+01:00")    >  utcnow("2022-08-01T13:51:30.000000Z")  # False 🥇
utcnow("2022-08-01T13:51Z")            >  utcnow("2022-08-01T13:51:30.000000Z")  # False 😻

This shown the returned values from the utcnow calls, and for what the comparisons is actually evaluated on.

"2022-08-01T23:51:30.000000Z"          >  "2022-08-01T13:51:30.000000Z"          # True  🎉
"2022-08-01T14:00:10.000000Z"          >  "2022-08-01T13:51:30.000000Z"          # True  ✅
"2022-08-01T13:00:10.000000Z"          >  "2022-08-01T13:51:30.000000Z"          # False 🥇
"2022-08-01T13:51:00.000000Z"          >  "2022-08-01T13:51:30.000000Z"          # False 😻

Transformation examples

Some additional examples of timestamps and to what they whould be converted. Thre first three examples are from the RFC document.

import utcnow

# This represents 20 minutes and 50.52 seconds after the 23rd hour of April 12th, 1985 UTC.
utcnow.get("1985-04-12T23:20:50.52Z")           # "1985-04-12T23:20:50.520000Z"

# This represents 39 minutes and 57 seconds after the 16th hour of December 19th, 1996 with
# an offset of -08:00 from UTC (Pacific Standard Time).  Note that this is equivalent to
# 1996-12-20T00:39:57Z in UTC.
utcnow.get("1996-12-19T16:39:57-08:00")         # "1996-12-20T00:39:57.000000Z"

# This represents the same instant of time as noon, January 1, 1937, Netherlands time.
# Standard time in the Netherlands was exactly 19 minutes and 32.13 seconds ahead of UTC by
# law from 1909-05-01 through 1937-06-30.
utcnow.get("1937-01-01T12:00:27.87+00:20")      # "1937-01-01T11:40:27.870000Z"

# Examples of other formats of accepted inputs:
utcnow.get("2021-02-18")                        # "2021-02-18T00:00:00.000000Z"
utcnow.get("2021-02-18 01:00")                  # "2021-02-18T01:00:00.000000Z"
utcnow.get("2021-02-18 03:00+01:00")            # "2021-02-18T02:00:00.000000Z"
utcnow.get("2021-02-18-01:00")                  # "2021-02-18T01:00:00.000000Z"
utcnow.get("2021-02-18+01:00")                  # "2021-02-17T23:00:00.000000Z"
utcnow.get("2021-02-18T23:55")                  # "2021-02-18T23:55:00.000000Z"
utcnow.get("2021-02-18T23:55:10")               # "2021-02-18T23:55:10.000000Z"
utcnow.get("2021-02-18T23:55:10.0")             # "2021-02-18T23:55:10.000000Z"
utcnow.get("2021-02-18T23:55:10.0+05:00")       # "2021-02-18T18:55:10.000000Z"
utcnow.get("2021-02-18T23:55:10.0-05:00")       # "2021-02-19T04:55:10.000000Z"
utcnow.get("2021-02-18T23:55:10.550-05:00")     # "2021-02-19T04:55:10.550000Z"
utcnow.get("2021-02-18 23:55:10.550+05:00")     # "2021-02-18T18:55:10.550000Z"
utcnow.get("2021-02-18 23:55:10.550-01:00")     # "2021-02-19T00:55:10.550000Z"
utcnow.get("2021-02-28 10:10:59.123987+00:00")  # "2021-02-28T10:10:59.123987Z"
utcnow.get("2021-02-28 10:10:59.123987Z")       # "2021-02-28T10:10:59.123987Z"
utcnow.get("2021-02-28 10:10:59.123987 UTC")    # "2021-02-28T10:10:59.123987Z"

Installation

Like you would install any other Python package, use pip, poetry, pipenv or your weapon of choice.

pip install utcnow

To install with Protocol Buffers support, specify the protobuf extras.

pip install utcnow[protobuf]

Usage and examples

# Transform timestamps of many different formats to the same fixed length standard

from utcnow import utcnow
result = utcnow.get("1984-08-01 13:38")
# "1984-08-01T13:38:00.000000Z"

# RFC 3339 timestamp as input – dates and datetimes – UTC assumed if tz is left out

from utcnow import utcnow
result = utcnow.get("2077-10-27")
# "2077-10-27T00:00:00.000000Z"

# Simple exmple of converting a naive datetime value, assuming UTC

import datetime
from utcnow import utcnow
dt = datetime.datetime(1984, 8, 1, 13, 38, 0, 4711)
result = utcnow.get(dt)
# "1984-08-01T13:38:00.004711Z"

# for non-tz-aware datetimes, the same result would be returned by both:
# 1. utcnow.get(dt)
# 2. dt.strftime("%Y-%m-%dT%H:%M:%S.%fZ")

# It's also possible to convert datetime values with tz offsets to timestamp strings

import datetime
from utcnow import utcnow
tz_EDT = datetime.timezone(offset=datetime.timedelta(hours=-4))
dt = datetime.datetime(1997, 8, 4, 2, 14, tzinfo=tz_EDT)
result = utcnow.get(dt)
# "1997-08-04T06:14:00.000000Z"

# for timezone-aware datetimes, the same result would be returned by both:
# 1. utcnow.get(dt)
# 2. dt.astimezone(datetime.timezone.utc).strftime("%Y-%m-%dT%H:%M:%S.%fZ")

# Vice versa, transforming a timestamp string to a datetime object (with tzinfo set to UTC)

from utcnow import utcnow
result = utcnow.as_datetime("1984-08-01T13:38:00.123450Z")
# datetime.datetime(1984, 8, 1, 13, 38, 0, 123450, tzinfo=datetime.timezone.utc)

# You can even convert your value to a unix timestamp, if that ever would do you any good

from utcnow import utcnow
result = utcnow.as_unixtime("1984-08-01T13:38:00.123450Z")
# 460215480.12345

# And the other way around again, converting from a unix timestamp to our date-time strings

from utcnow import utcnow
result = utcnow.get(0)
# "1970-01-01T00:00:00.000000Z"

result = utcnow.get(1614403926.108192)
# "2021-02-27T05:32:06.108192Z"

# Example using a value from "arrow" – a popular date-time Python lib with large featureset

import arrow
from utcnow import utcnow
value = arrow.get("2021-04-30T07:58:30.047110+02:00")
# <Arrow [2021-04-30T07:58:30.047110+02:00]>

str(value)
# "2021-04-30T07:58:30.047110+02:00"

result = utcnow.get(value)
# "2021-04-30T05:58:30.047110Z"

# the same output as via utcnow can be returned in following ways, also directly arrow:
# 1. utcnow.get(value)
# 2. value.to("UTC").strftime("%Y-%m-%dT%H:%M:%S.%fZ")

# Getting the current server time in UTC as a date-time timestamp string

import utcnow
utcnow.utcnow()
# "2021-02-18T08:24:48.382262Z"

# Similar can be accomplished with datetime – these lines returns the same string value:
# 1. utcnow.utcnow()
# 2. str(utcnow)
# 3. str(utcnow.utcnow)
# 4. utcnow.get()
# 5. utcnow.utcnow.get()
# 6. datetime.datetime.now(datetime.timezone.utc).strftime("%Y-%m-%dT%H:%M:%S.%fZ")
# 7. datetime.datetime.utcnow().strftime("%Y-%m-%dT%H:%M:%S.%fZ")
# 8. datetime.datetime.now(datetime.timezone.utc).isoformat(timespec="microseconds").replace("+00:00", "Z")
# 9. datetime.datetime.utcnow().isoformat(timespec="microseconds") + "Z"

# Getting the current server time in UTC but with a modifier

import utcnow
utcnow.utcnow("now", "+10d")
# "2021-02-28T08:24:48.382262Z"

# Or getting the current time in UTC as a datetime object

from utcnow import utcnow
utcnow.as_datetime()
# datetime.datetime(2021, 2, 18, 8, 24, 48, 382262, tzinfo=datetime.timezone.utc)

# this is merely a convinience, as the same value would be returned by both:
# 1. utcnow.as_datetime()
# 2. datetime.datetime.now(datetime.timezone.utc)
# 3. datetime.datetime.utcnow().replace(tzinfo=datetime.timezone.utc)

# As described – current server timestamp as a RFC 3339 date-time timestamp in UTC

import utcnow
result = str(utcnow)
# "2021-02-18T08:24:48.382262Z"

# Easy way of adding the current date-time timestamp to a JSON response

import json
import utcnow
result = json.dumps({"timestamp": str(utcnow), "status": 200})
# '{"timestamp": "2021-02-18T08:24:48.382262Z", "status": 200}'

# Or just adding the current time in an f-string

import utcnow
result = f"Current server time is: '{utcnow}'"
# "Current server time is: '2021-02-18T08:24:48.382262Z'"

Converting from Protocol Buffers message

It's also possible to transform a value encoded as a google.protobuf.Timestamp protobuf message in the same way you would from any other value.

# Using a google.protobuf.Timestamp message as input to utcnow
import utcnow
from google.protobuf.timestamp_pb2 import Timestamp
msg = Timestamp(seconds=1670329924, nanos=170660000)
result = utcnow.get(msg)
# "2022-12-06T12:32:04.170660Z"

# Using the binary data from a google.protobuf.Timestamp message
import utcnow
protobuf_msg_binary = b"\x08\xc4\xec\xbc\x9c\x06\x10\xa0\xa1\xb0Q"
result = utcnow.get(protobuf_msg_binary)
# "2022-12-06T12:32:04.170660Z"

You can also generate a new google.protobuf.Timestamp message using utcnow.as_protobuf()

import utcnow
msg = utcnow.as_protobuf("1984-08-01 22:30:47.234003Z")
# <class 'google.protobuf.timestamp_pb2.Timestamp'>
# · seconds: 460247447
# · nanos: 234003000

Note that the protobuf package has to be installed to make use of the above functionality. For convenience, it's also possible to install utcnow with protobuf support using the protobuf extras.

pip install utcnow[protobuf]

Get the date part as a string from a timestamp

Not as common, but every now and then you might need to get the date part from a timestamp (or for example today's date), to use in some string concatenation, S3 object keys and what not.

There's the long away around it, by generating a timestamp with utcnow.get() and then just keeping the first 10 characters of the timestamp – that's the date – YYYY-MM-DD. You could even use datetime and go datetime.datetime.now(datetime.timezone.utc).date().isoformat(), but it's not super clean.

utcnow comes with a wrapper function utcnow.as_date_string(value) to fetch just the date part based on the input value's UTC timestamp. Note that the date string that is returned does not include timezone information.

import utcnow

timestamp = "2022-04-05T13:44:52.748994Z"
utcnow.as_date_string(timestamp)
# "2022-04-05"

Bonus 🎉🎉 – calling the utcnow.as_date_string() function without arguments will return today's date, based on the current time in UTC. For some sugar to your code, the same function is also available under the name utcnow.today().

To get the current date in another timezone use the keyword argument tz to the function call. The value for tz should be either a datetime.tzinfo object or an utcoffset represented as a string (for example "+01:00", "-06:00", etc.).

import utcnow

utcnow.today()
# "2022-04-05" (it's the 5th of April when typing this)

utcnow.today(tz="+12:00")
# "2022-04-06" (time in UTC is currently 15:12 - adding 12 hours to that would yield 03:12 tomorrow)

How much time between timestamp A and timestamp B?

The library also comes with a small utility function for calculating the number of seconds (usually) between two timestamps.It's called utcnow.timediff and works like this.

import utcnow

# Afternoon meeting first work day of the year – also way too long
begin = "2021-01-04T13:00:00.000000Z"
end = "2021-01-04T17:30:00.000000Z"

seconds = utcnow.timediff(begin, end)
# 16200.0

# Additionally a unit can be specified as the third argument which automatically
# just divides the number of seconds with the value relative to the unit. If not
# specified, the default unit "seconds" will be applied.

seconds = utcnow.timediff(begin, end, "seconds")  # diff in seconds (alias: "s")
# 16200.0

minutes = utcnow.timediff(begin, end, "minutes")  # diff in minutes (alias: "m")
# 270.0

hours = utcnow.timediff(begin, end, "hours")  # diff in hours (alias: "h")
# 4.5

days = utcnow.timediff(begin, end, "days")  # diff in days (alias: "d")
# 0.1875

milliseconds = utcnow.timediff(begin, end, "milliseconds")  # diff in milliseconds (alias: "ms")
# 162000000.0

milliseconds = utcnow.timediff(begin, end, "microseconds")  # diff in microseconds (alias: "us")
# 16200000000.0

nanoseconds = utcnow.timediff(begin, end, "nanoseconds")  # diff in nanoseconds (alias: "ns")
# 16200000000000.0

import utcnow

# Another stupid example. How many seconds were there between unixtime epoch
# and unixtime 1234567890.
answer = utcnow.timediff(0, 1234567890)
# 1234567890.0

# This can also be calculated by using the power of subtraction.
also_the_answer = 1234567890 - 0
# 1234567890

Freeze the current time in utcnow with utcnow.synchronizer

There's a context manager available at utcnow.synchronizer to freeze the current time of utcnow to a specific value of your choice or to the current time when entering the context manager.

This has been added to accomodate for the use-case of needing to fetch the current time in different places as part of a call chain, where it's also either difficult or you're unable to pass the initial timestamp value as an argument down the chain, but you want to receive the exact same timestamp from utcnow to be returned for each call, although some microseconds would have passed since last call.

timestamp_before = utcnow.get()

with utcnow.synchronizer:
    # the current time (fetched through utcnow) is now frozen to the time when the
    # context manager was opened.
    timestamp_in_context = utcnow.get()

    # even when sleeping or awaiting, the time will stay frozen for as long as
    # we haven't exited the context.
    sleep(1)
    timestamp_1s_later = utcnow.get()  # same value as timestamp_in_context

timestamp_after = utcnow.get()

# timestamp_before      -> '2023-09-25T22:53:04.733040Z'
# timestamp_in_context  -> '2023-09-25T22:53:04.733076Z' (timestamp_before + 36µs)
# timestamp_1s_later    -> '2023-09-25T22:53:04.733076Z' (timestamp_before + 36µs)
# timestamp_after       -> '2023-09-25T22:53:05.738224Z' (timestamp_before + ~1s)

The utcnow.synchronizer(value, modifier) can also be initialized with a specific timestamp value to freeze the current time to, instead of the current time when entering the context manager. The same kind of arguments as for utcnow.rfc3339_timestamp() (utcnow.get()) can be used also for utcnow.synchronizer.

with utcnow.synchronizer("2000-01-01"):
    # the current time (fetched through utcnow) is now frozen to UTC midnight,
    # new years eve 2000.

    timestamp_in_context = utcnow.get()
    # '2000-01-01T00:00:00.000000Z'

    datetime_in_context = utcnow.as_datetime()
    # datetime.datetime(2000, 1, 1, 0, 0, tzinfo=datetime.timezone.utc)

    unixtime_in_context = utcnow.as_unixtime()
    # 946684800.0

    timestamp_in_context_plus_24h = utcnow.get("now", "+24h")
    # '2000-01-02T00:00:00.000000Z'

An example of how utcnow.synchronizer can be used to freeze the creation time of an object and its children

A common use-case for utcnow.synchronizer is to freeze the creation time of an object and its children, for example when creating a bunch of objects in a chunk, if we are expected to apply the exact same creation time of all objects in the chunk using the same value.

SomeModel is a simple class that stores created_at when the object is initiated. Objects of the type may also contain a list of items, which in pratice is children of the same type. If creating a parent SomeModel object with two children stored to its items list all together in a chunk (for example as part of a storage API request), we may want to use the same timestamp for all three objects (the parent and the children).

class SomeModel:
    _created_at: str
    items: List[SomeModel]

    def __init__(self, items: Optional[List[SomeModel]] = None) -> None:
        self._created_at = utcnow.rfc3339_timestamp()
        self.items = items if items is not None else []

    @property
    def created_at(self) -> str:
        return self._created_at

    def __repr__(self) -> str:
        base_ = f"{type(self).__name__} [object at {hex(id(self))}], created_at='{self._created_at}'"
        if self.items:
            return f"<{base_}, items={self.items}>"
        return f"<{base_}>"


# without freezing the current time, the timestamps would be different for each item and
# the parent although they were created in the same chunk - this may be desired in a bunch
# of cases, but not always.

a = SomeModel(items=[
    SomeModel(),
    SomeModel(),
])
# a = <SomeModel [object at 0x103a01350], created_at='2023-09-25T23:35:50.371100Z', items=[
#     <SomeModel [object at 0x1039ff590], created_at='2023-09-25T23:35:50.371078Z'>,
#     <SomeModel [object at 0x103a01290], created_at='2023-09-25T23:35:50.371095Z'>
# ]>

with utcnow.synchronizer:
    b = SomeModel(items=[
        SomeModel(),
        SomeModel(),
    ])
# b = <SomeModel [object at 0x103a01350], created_at='2023-09-25T23:35:50.371100Z', items=[
#     <SomeModel [object at 0x1039ff590], created_at='2023-09-25T23:35:50.371100Z'>,
#     <SomeModel [object at 0x103a01290], created_at='2023-09-25T23:35:50.371100Z'>
# ]>

It's not possible to chain utcnow.synchronizer context managers to freeze the current time to different values at different points in the call chain. If a utcnow.synchronizer context is already opened a second attempt to create or open a context will result in a raised exception.

with utcnow.synchronizer:
    # this is ok
    with utcnow.synchronizer:
        # we'll never get here
        ...

# Traceback (most recent call last):
#   File "<stdin>", line 2, in <module>
#   File ".../.../.../utcnow/__init__.py", line 245, in __enter__
#     raise RuntimeError("'utcnow.synchronizer' context cannot be nested (library time already synchronized)")
# RuntimeError: 'utcnow.synchronizer' context cannot be nested (library time already synchronized)

Finally

This is not a fullblown date library at all – it's lightweight, convenient utility package and should mostly just be used to output the date-time timestamp we want and at times convert values from other parts into our fixed length string format YYYY-MM-DDTHH:mm:ss.ffffffZ (or as %Y-%m-%dT%H:%M:%SZ as if used with datetime.datetime.strftime on a naive datetime value or a datetime value in UTC). Always uses UTC in output and always appends the UTC timezone as a Z to the string (instead of using +00:00, which a tz-aware datetime would do when used with datetime.isoformat()).

Use utcnow when you need to store date-time timestamps (and also preferably internet date-time timestamps) in any datastore as a string, using them in API responses (JSON responses usually) or when your services are adding timestamps in their log outputs.

Wether you choose to use this library or anything else, or just specify this is how we do it in a documement, it'll be worth it. It's never too late to start aligning your formatting standards and interfaces.

TLDR?

Use utcnow if you have a hard time being consistent with timestamp values in API:s and logging.

If you don't want to use utcnow, then here's a few key takeaways to remember.

• Always include timezones timezone as UTC when storing a timestamp (to database, within logging, everywhere).
• Always include timezones timezone as UTC when sending API responses.
• Set strict guidelines of how timestamps must be formatted within databases, in log output and API responses. Follow them.
• If your API accepts users to submit timestamps using arbitrary timezones or without tz info, immediately convert the timestamps to UTC.