def manualReverse(list):
return list[::-1]
def reverse(list):
return list(reversed(list))
list = [2,3,5,7,9]
print manualReverse(list)
print reverse(list)
I just started learning Python. Can anyone help me with the below questions?
1.How come list[::-1] returns the reversed list?
2.Why does the second function throw me NameError: name 'reverse' is not defined?
[::-1] is equivalent to [::1], but instead of going left to right, the negative makes it go right to left. With a negative step of one, this simply returns all the elements in the opposite order. The whole syntax is called the Python Slice Notation.
The reason why 'reverse' is not defined is because you did not globally define it. It is a local name in the manualReverse function. You can un-indent the function so it is a global function.
def manualReverse(list):
return list[::-1]
def reverse(list):
return list(reversed(list))
By the way, it's never a good idea to name lists list. It will override the built-in type, including the function too, which you depend on ( list(reversed(list)) )
manualReverse will also successfully reverse a tuple and return a tuple, but reverse as shown will return the reverse of a tuple as a list. Better reverse code would be (assuming the list arg is renamed to seq): return type(seq)(reversed(seq)).list[::-1] utilizes a slice notation and returns all the elements but in reversed order.
Explain Python's slice notation
Here is a detailed explanation with examples - it will answer this and more similar questions.
Indentation of def reverse(list) makes it visible only inside manualReverse(list).
If You unindent it will become visible globally.
[::-1] is not magic syntax to reverse a list. It is an instance of slice notation which is a more general concept, and a real answer would explain that and/or refer to the documentation.Simply use the builtin function reversed
>>> reversed(my_list)
See http://docs.python.org/2/library/functions.html?highlight=reversed#reversed
reversed function is used correctly in the question, except for the error in using list as both a variable name and a builtin.
reverseis a local name insidemanualReverse. Un-indent it.def reverseso it lines up withdef ManualReverse- otherwise, you're defining a function inside a function.... indentation is important in Pythonlist. That will override the builtin namelist, and indeed will cause an error in your code above once you fix the indentation.