When Python can't read your code

Sooner or later every program has a mistake in it. When Python can't understand a line, it stops and prints an error message instead of running the program. Learning to read these messages saves a lot of guessing.

A syntax error

Syntax is the set of rules for how code must be written, like grammar for a language. Here the text is missing its quotes:

print(Good morning)

  File "main.py", line 1
    print(Good morning)
          ^^^^^^^^^^^^
SyntaxError: invalid syntax. Perhaps you forgot a comma?

Read the message from the top:

• File "main.py", line 1 says where the problem is: line 1 of your program (on this site, your program's file is called main.py).
• The next line shows that line of code, and the ^ marks point at the part Python couldn't read.
• The last line names the kind of error, SyntaxError, with Python's guess at the cause. Its guess here (a missing comma) is wrong, which happens: the real problem is the missing quotes. The line number and the marks are usually more reliable than the guess.

With a syntax error, Python runs nothing at all, not even the lines before the mistake.

A name Python doesn't know

Here every quote is in place, but the command is misspelled:

print("Starting...")
Print("Hello")

Starting...

Traceback (most recent call last):
  File "main.py", line 2, in <module>
    Print("Hello")
    ^^^^^
NameError: name 'Print' is not defined. Did you mean: 'print'?

This time the first line ran and printed Starting..., and the program stopped at line 2. A NameError means Python doesn't know the name: Print with a capital P isn't print. The "Did you mean" part is often exactly right. On this site, a plain-English note under such a message explains it too.