[python] How to resolve TypeError: can only concatenate str (not "int") to str

Use f-strings to resolve the TypeError

• f-Strings: A New and Improved Way to Format Strings in Python
• PEP 498 - Literal String Interpolation

# the following line causes a TypeError
# test = 'Here is a test that can be run' + 15 + 'times'

# same intent with a f-string
i = 15

test = f'Here is a test that can be run {i} times'

print(test)

# output
'Here is a test that can be run 15 times'

i = 15
# t = 'test' + i  # will cause a TypeError

# should be
t = f'test{i}'

print(t)

# output
'test15'

• The issue may be attempting to evaluate an expression where a variable is the string of a numeric.
• Convert the string to an int.
• This scenario is specific to this question
• When iterating, it's important to be aware of the dtype

i = '15'
# t = 15 + i  # will cause a TypeError

# convert the string to int
t = 15 + int(i)
print(t)

# output
30

Note

• The preceding part of the answer addresses the TypeError shown in the question title, which is why people seem to be coming to this question.
• However, this doesn't resolve the issue in relation to the example provided by the OP, which is addressed below.

Original Code Issues

• TypeError is caused because message type is a str.
• The code iterates each character and attempts to add char, a str type, to an int
• That issue can be resolved by converting char to an int
• As the code is presented, secret_string needs to be initialized with 0 instead of "".
• The code also results in a ValueError: chr() arg not in range(0x110000) because 7429146 is out of range for chr().
• Resolved by using a smaller number
• The output is not a string, as was intended, which leads to the Updated Code in the question.

message = input("Enter a message you want to be revealed: ")

secret_string = 0

for char in message:
    char = int(char)
    value = char + 742146
    secret_string += ord(chr(value))
    
print(f'\nRevealed: {secret_string}')

# Output
Enter a message you want to be revealed:  999

Revealed: 2226465

Updated Code Issues

• message is now an int type, so for char in message: causes TypeError: 'int' object is not iterable
• message is converted to int to make sure the input is an int.
• Set the type with str()
• Only convert value to Unicode with chr
• Don't use ord

while True:
    try:
        message = str(int(input("Enter a message you want to be decrypt: ")))
        break
    except ValueError:
        print("Error, it must be an integer")
        
secret_string = ""
for char in message:
    
    value = int(char) + 10000
    
    secret_string += chr(value)

print("Decrypted", secret_string)

# output
Enter a message you want to be decrypt:  999
Decrypted ???

Enter a message you want to be decrypt:  100
Decrypted ???