1 | initial version |

This is a very partial answer, but in a Python crash course (by William Stein I presume, now I can't find the link) I learned this example:

```
def f(a, L=[]):
L.append(a)
return L
```

with the following astonishing outputs:

```
sage: f(2)
[2]
sage: f(5)
[2, 5]
sage: f(4)
[2, 5, 4]
```

(this is pure Python, Sage is not guilty).

2 | No.2 Revision |

This is a very partial answer, but in a Python crash course (by William Stein I presume, now I can't find the link) I learned this example:

```
def f(a, L=[]):
L.append(a)
return L
```

with the following astonishing successive outputs:

```
sage: f(2)
[2]
sage: f(5)
[2, 5]
sage: f(4)
[2, 5, 4]
```

(this is pure Python, Sage is not guilty).

Copyright Sage, 2010. Some rights reserved under creative commons license. Content on this site is licensed under a Creative Commons Attribution Share Alike 3.0 license.