Python built‑in functions: sorted, eval, and upper – syntax, parameters, and practical examples

The previous chapter introduced dict , filter , and lambda ; this chapter continues with additional functions useful for sorting dictionaries and other data structures.

sorted can sort any iterable. Unlike the list method list.sort() , which modifies the list in place, sorted() returns a new list. Its signature is sorted(iterable[, cmp[, key[, reverse]]]) where:

iterable – the collection to sort.

cmp – a two‑argument comparison function returning 1, -1, or 0.

key – a one‑argument function that extracts a comparison key from each element.

reverse – if True , sort in descending order.

Example using the default behavior:

a = [5, 7, 6, 3, 4, 1, 2]
b = sorted(a)  # b is [1, 2, 3, 4, 5, 6, 7]

Sorting with a custom cmp function:

L = [('b', 2), ('a', 1), ('c', 3), ('d', 4)]
b = sorted(L, cmp=lambda x, y: cmp(x[1], y[1]))

Sorting with a key function:

b = sorted(L, key=lambda x: x[1])

Sorting a list of student records by age using key and optionally reverse :

students = [('john', 'A', 15), ('jane', 'B', 12), ('dave', 'B', 10)]
sorted_by_age = sorted(students, key=lambda s: s[2])
sorted_desc = sorted(students, key=lambda s: s[2], reverse=True)

Using key and reverse together is generally faster than a custom cmp because the key function is called once per element.

eval evaluates a string as a Python expression and returns the result. It can convert string representations of lists, tuples, dictionaries, etc., into actual Python objects:

a = "[[1,2], [3,4], [5,6]]"
b = eval(a)  # b becomes [[1, 2], [3, 4], [5, 6]]
print(type(b))  #
c = "{1: 'a', 2: 'b'}"
d = eval(c)  # d becomes {1: 'a', 2: 'b'}
print(type(d))  #

Because eval executes arbitrary code, it poses serious security risks. Malicious input such as __import__('os').system('dir') can run system commands. Safer alternatives include writing custom validation functions or using ast.literal_eval , which only evaluates Python literals.

upper converts all lowercase letters in a string to uppercase. It takes no arguments and returns a new string:

str = "this is string example from runoob....wow!!!"
print("str.upper() : ", str.upper())  # STR.UPPER() :  THIS IS STRING EXAMPLE FROM RUNOOB....WOW!!!

The next chapter will cover the hashlib.md5 module.