Kelvin Ye - 10 months ago 58

Python Question

My first day in Python and get confused with a very short example. Hope anyone can provide some explanation about why there is some difference between these several versions. Please!

`a, b = 0, 1`

while b < 10:

print(b)

a, b = b, a+b

`a, b = 0, 1`

while b < 10:

print(b)

a = b

b = a+b

Answer Source

In the first version, the right hand is evaluated first, so `b`

hasn't been incremented when you add it.

To step through the first version for a couple iterations:

1.

```
a = 0
b = 1
a, b = 1, 1 # b is 1, and a is 0
```

2.

```
a = 1
b = 1
a, b = 1, 2 # b is 1 and a is 1
```

3.

```
a = 1
b = 2
a, b = 2, 3 # b is 2 and a is 1
```

In the second version, `b`

is assigned before you add it, so here's how the second version goes:

1.

```
a = 0
b = 1
a = b # a is now 1.
b = a + b # b is now 2, because both a and b are 1.
```

2.

```
a = 1
b = 2
a = b # a is now 2.
b = a + b # b is now 4, because both a and b are 2.
```