The "pit" and "pit" when a list is set as a parameter in a Python function

In the Python function, if the passed parameter is a list by default , then be aware that there is a pit here!!

Into the pit

def F (x,li=[]):    for in  Range (x):        li.append (i*i)     printprint('---1---') F (4)  Print('---2---') F (5)

Expected results

---1---1, 4, 9]---2---1, 4, 9, 16]

Execution results

---1---1, 4, 9]---2---1, 4, 9, 0, 1, 4, 9, 16]

Out of the Pit

When the function is defined, the value of the default parameter list in the function is saved, which is the list li=[];

If a new list is passed at each invocation, the passed list is used, no pass, and the default parameter (li=[] is saved when the function is defined);

In the above two calls, no new list is passed (using the default list li=[]), and the program invokes the default parameters ((li=[]) that are saved when the function is defined.

When the list is append, it will append append value on the li=[] original, so the result will be generated.

identification by the ID of the printed list

Print List li=[] ID:

def F (x,li=[]):    print(ID (li))  # Add print ID    for in Range (x):        li.append (i*i)    print(li)  print( ' ---1--- ' ) F(4)print('---2---') F (5)

Results:

---1---1403061239062481, 4, 9]---2---1403061239062481, 4, 9, 0, 1, 4, 9, 16]

Will find the ID value is the same;

Describes the default parameters used when defining a function when executing two times li=[]

Upload a new list in the execution

The ID of the print List li=[] and the ID of the new list passed in:

defF (x,li=[]):    Print(ID (LI)) forIinchRange (x): Li.append (i*i)Print(LI)Print('---1---') F (4)Print('---2---') F (5,[])Print('---3---') F (6)

Results:

---1---1, 4, 9]---2---1, 4, 9, +]---3---1, 4, 9, 0, 1, 4, 9, 16, 25]

You will find that a function that passes the empty (new) list does not print the same ID as it was passed;

When an empty list is passed, the empty list passed in the function body is used, and the function default value li=[] is not passed, so this result is generated.

Optimization

If you want to achieve the desired result, simply make a judgment in the function body:

defF (x, li=[]):    if  notLi:#If Li is not empty then go down (empty list);Li = []     forIinchRange (x): Li.append (i*i)Print(LI)Print('---1---') F (4)Print('---2---') F (5)Print('---3---') F (6)

Results:

---1---1, 4, 9]---2---1, 4, 9, +]---3---1, 4, 9, 16, 25]

The "pit" and "pit" when a list is set as a parameter in a Python function