When first learning Python, trying to understand the meaning of Python's error messages can be a bit complicated. Here is a list of common runtime errors that can crash your program.
1) Forgot to add: at the end of if , elif , else , for , while , class , def declarations (resulting in " SyntaxError: invalid syntax ")
The error will occur in code similar to the following:
if spam == 42 print('Hello!')
2) Use = instead of == (results in " SyntaxError: invalid syntax ")
= is an assignment operator and == is an equality comparison. The error occurs in the following code:
if spam = 42: print('Hello!')
3) Wrong use of indentation. (resulting in " IndentationError: unexpected indent ", " IndentationError: unindent does not match any outer indetation level ", and " IndentationError: expected an indented block ")
Remember that indentation increments are only used after statements ending in :, and must then revert to the previous indentation format. The error occurs in the following code:
print('Hello!') print('Howdy!') Or: if spam == 42 : print ( ' Hello! ' ) print ( ' Howdy! ' ) or: if spam == 42 : print ( ' Hello! ' )
4) Forgot to call len() in the for loop statement (resulting in " TypeError: 'list' object cannot be interpreted as an integer ")
Often you want to iterate over the elements of a list or string by index, which requires calling the range() function. Remember to return the len value instead of the list.
The error occurs in the following code:
spam = ['cat', 'dog', 'mouse']for i in range(spam): print(spam[i])
5) Attempt to modify the value of string (resulting in " TypeError: 'str' object does not support item assignment ")
string is an immutable data type and the error occurs in code like this:
spam = 'I have a pet cat.'spam[13] = 'r'print(spam)
And you actually want to do this:
spam = 'I have a pet cat.'spam = spam[:13] + 'r' + spam[14:] print(spam)
6) Attempt to concatenate non-string value with string (results in " TypeError: Can't convert 'int' object to str implicitly ")
The error occurs in the following code:
numEggs = 12print('I have ' + numEggs + ' eggs.')
And you actually want to do this:
numEggs = 12print('I have ' + str(numEggs) + ' eggs.')
or:
numEggs = 12print('I have %s eggs.' % (numEggs))
7) Forgetting to put quotes at the beginning and end of the string (resulting in " SyntaxError: EOL while scanning string literal ")
The error occurs in the following code:
print(Hello!')
or:
print('Hello!)
or:
myName = 'Al'print('My name is ' + myName + . How are you?')
8) The variable or function name is misspelled (resulting in " NameError: name 'fooba' is not defined ")
The error occurs in the following code:
foobar = 'Al'print('My name is ' + fooba)
or:
spam = ruond (4.2)
or:
spam = Round(4.2)
9) Method name misspelled (resulting in " AttributeError: 'str' object has no attribute 'lowerr '")
The error occurs in the following code:
spam = 'THIS IS IN LOWERCASE.'spam = spam.lowerr()
10) The reference exceeds the maximum index of the list (resulting in " IndexError: list index out of range ")
The error occurs in the following code:
spam = ['cat', 'dog', 'mouse'] print(spam[6])
11) Using a dictionary key that doesn't exist (results in "KeyError: 'spam'")
The error occurs in the following code:
spam = {'cat': 'Zophie', 'dog': 'Basil', 'mouse': 'Whiskers'}
print('The name of my pet zebra is ' + spam['zebra'])
12) Attempting to use Python keywords as variable names (results in " SyntaxError: invalid syntax ")
Python keys cannot be used as variable names, the error occurs in code like this:
class = 'algebra'
Python3的关键字有：and, as, assert, break, class, continue, def, del, elif, else, except, False, finally, for, from, global, if, import, in, is, lambda, None, nonlocal, not, or, pass, raise, return, True, try, while, with, yield
13) Using the increment operator in a variable that defines a new variable (results in " NameError: name 'foobar' is not defined ")
Do not use 0 or an empty string as an initial value when declaring a variable, so that a sentence using the auto-increment operator spam += 1 is equal to spam = spam + 1, which means that spam needs to specify a valid initial value.
The error occurs in the following code:
spam = 0spam += 42eggs += 42
14) Use a local variable in a function before defining a local variable (there is a global variable with the same name as the local variable at this time) (resulting in " UnboundLocalError: local variable 'foobar' referenced before assignment ")
It's complicated to use a local variable in a function while there is a global variable of the same name. The rule of use is: if anything is defined in the function, if it's only used in the function then it's local, otherwise it's global variable.
This means you can't use it as a global variable in a function before defining it.
The error occurs in the following code:
someVar = 42def myFunction(): print(someVar) someVar = 100myFunction()
15) Attempting to use range() to create a list of integers (results in " TypeError: 'range' object does not support item assignment ")
Sometimes you want to get an ordered list of integers, so range() seems like a good way to generate this list. However, you need to remember that range() returns a "range object", not an actual list value.
The error occurs in the following code:
spam = range(10)
spam[4] = -1
Maybe this is what you want to do:
spam = list(range(10))
spam[4] = -1
(Note: spam = range(10) works in Python 2 because range() returns list values in Python 2, but in Python 3 the above error will occur)
16) Nice in ++ or — increment and decrement operators. (results in " SyntaxError: invalid syntax ")
If you're used to other languages like C++, Java, PHP, etc., maybe you'll want to try using ++ or - incrementing/decrementing a variable. There is no such operator in Python.
The error occurs in the following code:
spam = 1spam++
Maybe this is what you want to do:
spam = 1spam += 1
17) Forgot to add the self parameter to the first parameter of the method (resulting in "TypeError: myMethod() takes no arguments (1 given)")
The error occurs in the following code:
class Foo(): def myMethod(): print('Hello!') a = Foo() a.myMethod()