Python技术问询：如何截取字符串字符、去除datetime输出的毫秒

Hey there! Let's break down your two Python questions clearly, especially since you mentioned you're a total beginner. 😊

The go-to way to grab parts of a string in Python is slicing—it's super flexible and easy to use once you get the hang of the syntax:

your_string[start_index:end_index:step]

Here's what each part means:

• start_index: Optional, defaults to 0 (the first character of the string)
• end_index: Optional, defaults to the end of the string, but note that this character isn't included in the result
• step: Optional, defaults to 1 (meaning we take every character in between)

Let's use examples to make this concrete:

my_string = "Hello Python Newbie"
# Grab the first 5 characters
print(my_string[:5])  # Output: Hello
# Grab everything from index 6 onwards
print(my_string[6:])  # Output: Python Newbie
# Grab just "Python" (from index 6 to 12, not including 12)
print(my_string[6:12])  # Output: Python
# Grab the last 6 characters using negative indexing
print(my_string[-6:])  # Output: Newbie
# Grab every other character
print(my_string[::2])  # Output: Hlo yhn eie

If you need to split the string by a specific separator (like commas or spaces) first, you can use the split() method, then pick the part you want:

fruits = "apple,banana,cherry"
fruit_list = fruits.split(",")
print(fruit_list[1])  # Output: banana

Great job getting the current time to print successfully! To get rid of those milliseconds, here are two solid methods:

Method 1: Use strftime() for formatted output (Recommended) #

The datetime object has a built-in strftime() method that lets you define exactly how you want your time to look. You'll need these format codes:

• %Y: 4-digit year (e.g., 2018)
• %m: 2-digit month (e.g., 04)
• %d: 2-digit day (e.g., 05)
• %H: 24-hour format hour (e.g., 10)
• %M: Minute (e.g., 55)
• %S: Second (e.g., 36)

Here's your modified code:

from datetime import datetime
current_time = datetime.now()
# Print with your desired format—no milliseconds!
print(current_time.strftime("%Y-%m-%d %H:%M:%S"))

Method 2: String slicing (Quick but less reliable) #

If you don't want to memorize format codes, you can convert the datetime object to a string and just take the first 19 characters (since the default output starts with YYYY-MM-DD HH:MM:SS before the milliseconds):

from datetime import datetime
current_time = datetime.now()
# Slice the string to keep only the first 19 characters
print(str(current_time)[:19])

A quick heads-up: This depends on Python's default string format for datetime objects. If that ever changes in a future version, this method might break. So Method 1 is the safer bet long-term.

内容的提问来源于stack exchange，提问作者vikino