Reading a List with eval()

1The program

# read the whole list in one go

numbers = eval(input('Enter a list: '))

print('The list is', numbers)
print('It has', len(numbers), 'items')
print('The first item is', numbers[0])

Enter a list: [10, 20, 30]
The list is [10, 20, 30]
It has 3 items
The first item is 10

One question instead of four. But the interesting part is what would have happened without the eval(), because that is the part the program hides:

# the same input, without eval()

numbers = input('Enter a list: ')

print('The value is', numbers)
print('Its length is', len(numbers))
print('Its first item is', numbers[0])
print('Its type is', type(numbers))

Enter a list: [10, 20, 30]
The value is [10, 20, 30]
Its length is 12
Its first item is [
Its type is <class 'str'>

2What eval() actually does

eval() takes a string, reads it as though you had typed it into a Python program, works it out, and hands back the answer. It is not about lists at all:

# eval() runs whatever the text says

print(eval('2 + 3'))
print(eval('10 * 4'))
print(type(eval('[1, 2, 3]')))
print(type(eval('10, 20, 30')))

5
40
<class 'list'>
<class 'tuple'>

3The two ways, side by side

Step both programs forward together. Watch the left one grow an item at a time, and watch the right one pass through a piece of text on its way to becoming a list.

4Three ways eval() bites

1. Forget the brackets and you do not get a list. The program runs, everything looks right, and the type is wrong:

Enter a list: 10, 20, 30
The list is (10, 20, 30)
It has 3 items
The first item is 10

Round brackets in the output, not square: that is a tuple, and the very next thing the program tries to do — append(), or numbers[0] = 5 — will fail, because a tuple cannot be changed.

2. A typo stops the program. Miss a bracket and the text is not legal Python, so eval() refuses it:

(the same program, with a bracket missing)

Enter a list: [10, 20, 30
Traceback (most recent call last):
  File "read_eval.py", line 1, in <module>
    numbers = eval(input('Enter a list: '))
              ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "<string>", line 1
    [10, 20, 30
    ^
SyntaxError: '[' was never closed

Type a bare word and you get a different refusal, because Python reads it as the name of a variable that does not exist:

(the same program, with a name typed instead of a list)

Enter a list: Asha
Traceback (most recent call last):
  File "read_eval.py", line 1, in <module>
    numbers = eval(input('Enter a list: '))
              ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "<string>", line 1, in <module>
NameError: name 'Asha' is not defined

3. It runs whatever it is given. This is the serious one. eval() does not check that the text is a list — it runs it, whatever it is. Anything the user could type into a Python program, they can type into that prompt, and it will be carried out. For a classroom exercise that is fine. For a program handling other people's data it is a real problem, which is why professional code avoids eval() on anything a stranger typed.

5Which one should you use?

# a third way: split one line at the spaces

text = input('Enter the numbers, separated by spaces: ')
pieces = text.split()
numbers = []

for p in pieces:
    numbers.append(int(p))

print('As text: ', pieces)
print('As numbers:', numbers)

Enter the numbers, separated by spaces: 10 20 30
As text:  ['10', '20', '30']
As numbers: [10, 20, 30]

numbers = eval(input('Enter a list: '))

print('The list is', numbers)
print('It has', len(numbers), 'items')
print('The first item is', numbers[0])

6Recap