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ValueError: invalid literal for int() with base 10

ValueError: invalid literal for int() with base 10

DodaTech 3 min read

ValueError: invalid literal for int() with base 10 occurs when you pass a string to int() that doesn’t look like a valid integer. The string must contain only digits (with optional leading + or -).

What It Means

The int() function expects a string like "42" or "-7" but received something else — maybe "42.5", "4,2", "abc", or " 42 " with whitespace. Any non-digit character (except a leading sign) causes this error.

Why It Happens

  • String contains whitespace: " 42 " (note: int(" 42") actually works in Python 3, but int("42 ") fails)
  • String has commas or decimal points: "4,200" or "3.14"
  • String is empty: ""
  • String is not numeric: "abc", "ten"
  • String has leading zeros that don’t affect parsing but might indicate bad data
  • User input wasn’t validated before conversion

How to Fix It

Step 1: Strip whitespace and non-digit characters 

# BAD
user_input = " 42 "
value = int(user_input)  # Some of these may work, but be explicit

# GOOD — strip first
value = int(user_input.strip())

Step 2: Remove commas and decimal points 

price = "4,200"

# BAD
int(price)  # ValueError

# GOOD — remove commas first
value = int(price.replace(",", ""))

# For floats, convert to float first, then int
decimal_str = "3.14"
value = int(float(decimal_str))  # 3

Step 3: Validate before converting 

user_input = input("Enter a number: ")

if user_input.isdigit() or (user_input.startswith("-") and user_input[1:].isdigit()):
    value = int(user_input)
else:
    print(f"'{user_input}' is not a valid integer")

Step 4: Use try/except for robust conversion 

def safe_int(value, default=None):
    try:
        return int(value)
    except (ValueError, TypeError):
        return default

# Usage
result = safe_int("42")       # 42
result = safe_int("abc")      # None
result = safe_int("abc", 0)   # 0

Step 5: Handle empty strings 

user_input = input("Enter a number: ").strip()

if not user_input:
    print("Input cannot be empty")
else:
    try:
        value = int(user_input)
    except ValueError:
        print(f"'{user_input}' is not a valid integer")

Step 6: Fix common number format issues 

# Currency symbols
amount = "$42"
value = int(amount.replace("$", ""))  # 42

# Percentage
pct = "85%"
value = int(pct.replace("%", ""))  # 85

# Spaces around
text = " 42 "
value = int(text.strip())  # 42

# Negative sign with space
text = "- 42"
value = int(text.replace(" ", ""))  # -42
Does int() accept floats as input?
Yes, int(3.14) returns 3 — it truncates toward zero. But int("3.14") raises ValueError. If you’re parsing a string that might contain a decimal, convert to float first: int(float("3.14")).
What does 'base 10' mean in the error?
It means int() is trying to parse the string as a decimal (base 10) number. Python’s int() accepts an optional second argument for the base: int("FF", 16) returns 255. Base 10 assumes digits 0-9 only.
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