Cannot understand how this Python code works

I found this question on HackerRank and I am unable to understand the code(solution) that is displayed in the discussions page.

The question is:
Consider a list (list = ). You can perform the following commands:

insert i e: Insert integer at position .
print: Print the list.
remove e: Delete the first occurrence of integer .
append e: Insert integer at the end of the list.
sort: Sort the list.
pop: Pop the last element from the list.
reverse: Reverse the list.

Even though I have solved the problem using if-else, I do not understand how this code works:

n = input()
slist =
for _ in range(n):
s = input().split()
cmd = s[0]
args = s[1:]
if cmd !="print":
cmd += "("+ ",".join(args) +")"
eval("slist."+cmd)
else:
print slist


n = int(input())

3 Answers
3

Well, the code takes advantage of Python's eval function. Many languages have this feature: eval, short for "evaluate", takes a piece of text and executes it as if it were part of the program instead of just a piece of data fed to the program. This line:

eval

eval

s = input().split()


reads a line of input from the user and splits it into words based on whitespace, so if you type "insert 1 2", s is set to the list ["insert","1","2"]. That is then transformed by the following lines into "insert(1,2)", which is then appended to "slist." and passed to eval, resulting in the method call slist.insert(1,2) being executed. So basically, this code is taking advantage of the fact that Python already has methods to perform the required functions, that even happen to have the same names used in the problem. All it has to do is take the name and arguments from an input line and transform them into Python syntax. (The print option is special-cased since there is no method slist.print(); for that case it uses the global command: print slist.)

s

["insert","1","2"]

"insert(1,2)"

"slist."

eval

slist.insert(1,2)

print

slist.print()

print slist

In real-world code, you should almost never use eval; it is a very dangerous feature, since it allows users of your application to potentially cause it to run any code they want. It's certainly one of the easier features for hackers to use to break into things.

eval

It's dirty code that's abusing eval.

eval

Basically, when you enter, for example, "remove 1", it creates some code that looks like sList.remove(1), then gives the created code to eval. This has Python interpret it.

"remove 1"

sList.remove(1)

eval

This is probably the worst way you could solve this outside of coding competitions though. The use of eval is entirely unnecessary here.

eval

Actually I Find some error in the code, but I came to an understanding of how this code runs. here is it:

input :

3
1 2 3
cmd = 1 + ( 2 + 3)


then eval(cmd) i.e., eval("1 + (2 + 3)") which gives an output 6
another input:

eval(cmd)

eval("1 + (2 + 3)")

4
4 5 6 2
cmd = 4 + ( 5 + 6 + 2)
eval(cmd)


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