Pythonでタイムゾーンを使用する方法は？

Use zoneinfo for Python 3.9+ to create timezone-aware datetimes and convert between timezones with astimezone(); 2. For Python 3.6–3.8, use pytz with localize() to avoid DST errors; 3. Always work in UTC internally and convert to local time only for display; 4. Parse timezone-aware strings using fromisoformat() or dateutil.parser, and avoid naive and aware datetime comparisons, ensuring consistent, unambiguous time handling in applications.

Working with timezones in Python can be tricky, but with the right tools and understanding, it becomes manageable. The key is to always be aware of whether your datetime objects are naive (no timezone info) or aware (include timezone info), and to convert between timezones safely.

Here’s how to handle timezones properly in Python:

1. Use datetime with zoneinfo (Python 3.9+)

The modern and recommended way is to use the built-in zoneinfo module (available in Python 3.9+). It uses the IANA timezone database and doesn’t require external libraries.

from datetime import datetime
from zoneinfo import ZoneInfo

# Create a datetime with a specific timezone
local_time = datetime(2024, 4, 5, 12, 0, 0, tzinfo=ZoneInfo("America/New_York"))
print(local_time)  # 2024-04-05 12:00:00-04:00

# Convert to another timezone
utc_time = local_time.astimezone(ZoneInfo("UTC"))
print(utc_time)  # 2024-04-05 16:00:00+00:00

# Get current time in a timezone
now_in_london = datetime.now(ZoneInfo("Europe/London"))
print(now_in_london)

⚠️ Always use astimezone() to convert between timezones, not just assigning tzinfo.

2. For older Python versions (3.6–3.8): Use pytz

If you're on Python < 3.9, install and use pytz:

pip install pytz

from datetime import datetime
import pytz

# Define a timezone
tz_ny = pytz.timezone("America/New_York")

# Localize a naive datetime (correct way)
naive = datetime(2024, 4, 5, 12, 0, 0)
local_time = tz_ny.localize(naive)
print(local_time)  # 2024-04-05 12:00:00-04:00

# Convert to UTC
utc_time = local_time.astimezone(pytz.UTC)
print(utc_time)  # 2024-04-05 16:00:00+00:00

❗ Never do datetime(2024, 4, 5, 12, 0, tzinfo=tz_ny) — always use .localize() to avoid DST errors.

3. Work in UTC internally

Best practice: store and compute all timestamps in UTC, then convert to local time only for display.

from datetime import datetime
from zoneinfo import ZoneInfo

# Store time in UTC
utc_now = datetime.now(ZoneInfo("UTC"))

# Convert to user's timezone when displaying
user_tz = ZoneInfo("Asia/Tokyo")
japan_time = utc_now.astimezone(user_tz)
print(f"Current time in Tokyo: {japan_time}")

This avoids ambiguity, especially around daylight saving transitions.

4. Parse and format timezone-aware strings

Use strptime carefully — it may create naive datetimes.

from datetime import datetime
from zoneinfo import ZoneInfo

# Parsing ISO format with timezone
date_str = "2024-04-05T12:00:00+02:00"
dt = datetime.fromisoformat(date_str)
print(dt.tzinfo)  # Offset attached

# Or parse and assign timezone
raw_str = "2024-04-05 10:00:00"
naive = datetime.strptime(raw_str, "%Y-%m-%d %H:%M:%S")
aware = naive.replace(tzinfo=ZoneInfo("Europe/Paris"))

For more complex parsing, consider dateutil:

pip install python-dateutil

from dateutil import parser

dt = parser.parse("2024-04-05 10:00:00 EST")
print(dt)  # Already timezone-aware

Key Tips

• Always prefer aware datetime objects in production.
• Never compare naive and aware datetimes — it raises errors in Python 3.9+.
• Use standard timezone names like America/Los_Angeles, not abbreviations like PST (they’re ambiguous).
• Be careful with DST: .localize() and .astimezone() handle it correctly; direct tzinfo= does not.

Basically, use zoneinfo if you can, work in UTC, and always be explicit about timezones. It’s not hard once you get the pattern down.

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