Python ValueError

3ValueError: explained step by step

A ValueError means a function received a value of the right type but with unsuitable content. int() happily accepts strings, but not the string "abc". The type is fine; the value is the problem. Compare this with a TypeError, where the type itself is wrong, such as adding text to a number.

You will meet ValueError most often when you turn text into numbers (see type casting), unpack a sequence into the wrong number of names, or ask a function to do something it can't do with the data you gave it. Because it is an ordinary exception, you can catch it with except ValueError.

How to read a ValueError message

Traceback (most recent call last):
  File "main.py", line 2, in <module>
    quantity = int(answer)
ValueError: invalid literal for int() with base 10: 'three'

The last line has two parts: the function that complained (int()) and the exact value it rejected, shown with repr() quotes: 'three'. Those quotes are valuable. If you see '42 apples' or '', you instantly know there is extra text or nothing at all.

Why int() says "invalid literal for int() with base 10"

int() only accepts text that looks like a whole number. Letters, decimal points, commas, units and empty strings are all rejected. Leading and trailing spaces are fine.

for text in ["42", " 42 ", "3.5", "1,200", "12 kg", ""]:
    try:
        print(repr(text), "->", int(text))
    except ValueError as err:
        print("ValueError:", err)

To read a decimal string use float() first, then int() if you need a whole number. Remove thousands separators with text.replace(",", ""), and use .strip() to clean stray whitespace (see string methods).

Why float() says "could not convert string to float"

float() has the same rule with a different message. A European decimal comma ("1,5") or a currency sign ("$9.99") fails.

for text in ["9.99", "$9.99", "1,5", "abc"]:
    try:
        print(repr(text), "->", float(text))
    except ValueError as err:
        print("ValueError:", err)

print(float("$9.99".lstrip("$")))

Unpacking: "too many values" and "not enough values"

Unpacking assigns each item of a sequence to one name, so the counts must match.

try:
    first, last = "Asha Rao Singh".split()
except ValueError as err:
    print("ValueError:", err)

try:
    day, month, year = (21, 10)
except ValueError as err:
    print("ValueError:", err)

first, *middle, last = "Asha Rao Singh".split()
print(first, middle, last)

Count the values on the right, or use a starred name (*middle) to collect the extras.

Why list.remove(x) says "x not in list"

Methods such as remove() and index() raise ValueError when the item isn't there. (A dictionary lookup raises KeyError instead.) Check with in first, as covered in list methods.

cart = ["pen", "book"]
try:
    cart.remove("lamp")
except ValueError as err:
    print("ValueError:", err)

if "lamp" in cart:
    cart.remove("lamp")
print(cart)

Other everyday ValueErrors

The same family shows up in the standard library. Each message names its own problem:

import math
from datetime import datetime

attempts = [
    lambda: math.sqrt(-9),
    lambda: max([]),
    lambda: datetime.strptime("21/10/2026", "%Y-%m-%d"),
]
for attempt in attempts:
    try:
        attempt()
    except ValueError as err:
        print("ValueError:", err)

math.sqrt(-9) fails because real square roots of negatives don't exist (math domain error). max([]) has nothing to compare, so check the list is not empty first. strptime needs a format that matches the text exactly; see the datetime module and math functions.

Handling bad input with try/except ValueError

For user input, don't try to predict every bad string. Attempt the conversion and handle the failure. Here a list stands in for what a user might type (see input and output):

answers = ["abc", "", "-3", "7"]

for answer in answers:
    try:
        quantity = int(answer)
    except ValueError:
        print(f"{answer!r} is not a whole number, ask again")
        continue
    print("Accepted", quantity)
    break

Avoid answer.isdigit() as your only check: it returns False for "-3" and "2.5", which are valid numbers.

Raising ValueError yourself

When your own function receives a value that makes no sense, raise a ValueError with a clear message instead of returning a wrong result:

def set_age(age):
    if age < 0:
        raise ValueError(f"age cannot be negative, got {age}")
    return age

try:
    set_age(-4)
except ValueError as err:
    print("ValueError:", err)