对于以下python代码段的几行内容，需要说明。

Question

Below python code are used for perform operation like append,sort,print on the list L .I was analyzing the code, but unable to understand few lines in the code.

 l = []#declare the list
 choices = {"0": "", "1": "{}()", "2": "{}({})", "3": "{}({}, {})"}#dicationary declaration
 for _ in range(int(raw_input())):  #prompt for user input and iterate 
        cmds = raw_input().split(" ")   #get the operation that needs to be performed on the list
        choice = str(len(cmds))   #get the length
        if cmds[0] == "print": print l   #print the list
        elif choice in choices: eval("l." + choices[choice].format(*cmds))

Dictionary declaration, choices = {"0": "", "1": "{}()", "2": "{}({})", "3": "{}({}, {})"} have brackets and parenthesis, I am unable to understand its significance. The last line elif choice in choices: eval("l." + choices[choice].format(*cmds)) seems to be mysterious because of

1. eval function which is be used to execute the Python code.
2. string function format which seems to be obscure with the addition of symbols * .

Input are in the below format

insert 0 6
print 
remove 6
append 9

Answer 1

This is really ugly code. insert 0 6 is invalid as the first input; first you must state how many times you want to use this monstrosity eg 1 .

Then:

1. The input insert 0 6 has three parameters in it. cmds = raw_input.split(" ") and choice = str(len(cmds)) then works out that you passed three ( 3 ) parameters to the code.

2. The number 3 is then used to give back the appropriate formatting string from choices = {"0": "", "1": "{}()", "2": "{}({})", "3": "{}({}, {})"} . In this case; "{}({}, {})"

3. We didn't ask for a print , we asked for insert , so if cmds[0] == "print": print l is False and we skip it.

4. This means we then have to evaluate elif choice in choices: eval("l." + choices[choice].format(*cmds)) . Well, we already know from point (2) that choice == 3 corresponds to "{}({}, {})" . The (*cmds)) is for tuple unpacking... it unpacks (input, 0, 6) into the string at the curly braces, giving "insert(0, 6)" .
5. We then concatenate the string at the end of (4) to give "l.insert(0, 6)" as a string. That is then passed to eval and executed.

Answer 2

This function allows the user to input methods that work on a list as raw input. It then creates a string with that is a valid line of python code and evaluates it on the list.

The choices dictionary contains the format strings that are used to construct the line that will be evaluated. The brackets {} will be substituted by items from the input list in the .format call. The parenthesis () are the parenthesis used to make it a proper function call in python.

If you replace the eval with print you will see exactly what the command looks like after the substitutions.

Also note, this code only works for Python 2, for Python 3 you need to use:

l = []#declare the list
choices = {"0": "", "1": "{}()", "2": "{}({})", "3": "{}({}, {})"}#dicationary declaration
for _ in range(int(input())):  #prompt for user input and iterate 
    cmds = input().split(" ")   #get the operation that needs to be performed on the list
    choice = str(len(cmds))   #get the length
    if cmds[0] == "print": print(l)   #print the list
    elif choice in choices: eval("l." + choices[choice].format(*cmds))