Python 3中round(395,-2)的运行原理是什么？

Hey there! Let's break down exactly how Python's round() function works here, step by step.

First, a quick recap of the round() parameters #

The round() function takes two arguments:

• The first is the number you want to round
• The second (ndigits) tells Python how many digits after the decimal point to keep. But when this number is negative, it shifts focus to digits before the decimal point—this is the key to your example.

What does ndigits=-2 mean? #

When you pass -2 as the second argument, you're telling Python to round your number to the nearest multiple of 100 (since 10² = 100). Here's how it plays out for 395:

1. Identify the digit in the hundreds place (the position we're rounding to): that's 3 in 395
2. Look at the digits to the right of this position: 95 (the tens and ones place)
3. Since 95 is 50 or greater, we round the hundreds digit up by 1 (from 3 to 4)
4. Replace all digits to the right of the hundreds place with 0s, resulting in 400

The underlying math (if you want to dive deeper) #

You can think of it as a simple three-step calculation:

• Divide the original number by 10 raised to the absolute value of ndigits: 395 / 100 = 3.95
• Round that result to the nearest integer: round(3.95) = 4
• Multiply back by the same power of 10: 4 * 100 = 400

Quick examples to solidify this #

• round(245, -2) → 200 (since 45 < 50, we keep the hundreds digit as 2)
• round(550, -2) → 600 (550 is closer to 600 than 500, so it rounds up)
• round(250, -2) → 200 (this is an edge case: Python uses "bankers rounding" here, which rounds to the nearest even number when the value is exactly halfway between two multiples)

内容的提问来源于stack exchange，提问作者Satheesh