Summary of common mistakes made by beginners when learning Python

I have been learning Python recently, and now I have summarized some common mistakes as follows:

1) Forgot to put at the end of if, elif, else, for, while, class, def statement Add: (Causes "SyntaxError: invalid syntax") This error will occur in code similar to the following:

if spam == 42 print('Hello!')

2) Use = instead of == (Causes "SyntaxError: invalid syntax") = is the assignment operator And == is an equal comparison operation. This 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 increases only after statements that end with: and must be restored afterwards to the previous indentation format. This 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") Usually 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. This error occurs in the following code:

spam = ['cat', 'dog', 'mouse'] 
for i in range(spam): 
     print(spam[i])

5) Try to modify the value of string (resulting in "TypeError: 'str' object does not support item assignment") string is an immutable data type, this error Happens in code like:

spam = 'I have a pet cat.' 
spam[13] = 'r' print(spam)

whereas you actually want to do this:

spam = 'I have a pet cat.' 
spam = spam[:13] + 'r' + spam[14:] print(spam)

6) Try concatenating a non-string value with a string (resulting in "TypeError: Can't convert 'int ' object to str implicitly") The error occurs in code like:

numEggs = 12 print('I have ' + numEggs + ' eggs.')

whereas you actually want to do:

numEggs = 12 print('I have ' + str(numEggs) + ' eggs.')

or:

 numEggs = 12 print('I have %s eggs.' % (numEggs))

7) in the character Forgot to add quotation marks at the beginning and end of the string (resulting in "SyntaxError: EOL while scanning string literal") This error occurs in the following code:

print(Hello!') 或者: print('Hello!)

or:

myName = 'Al' print('My name is ' + myName + . How are you?')

8) The variable or function name is spelled incorrectly ( Causes "NameError: name 'fooba' is not defined") This error occurs in the following code:

foobar = 'Al' print('My name is ' + fooba)
 或者:
spam = ruond(4.2) 
或者: 
spam = Round(4.2)

9) The method name is spelled incorrectly (causes "AttributeError: 'str' object has no attribute 'lowerr'" ) This error occurs in the following code:

spam = 'THIS IS IN LOWERCASE.' 
spam = spam.lowerr()

10) The reference exceeds the maximum index of list (resulting in "IndexError: list index out of range") This error occurs in the following code:

spam = ['cat', 'dog', 'mouse'] 
print(spam[6])

11) Using a dictionary key value that does not exist (causing "KeyError: 'spam'") This error occurs in code like this:

spam = {'cat': 'Zophie', 'dog': 'Basil', 'mouse': 'Whiskers'}
print('The name of my pet zebra is ' + spam['zebra'])

12) Trying to use a Python keyword as a variable name (causing "SyntaxError: invalid syntax") Python key cannot be used as a variable name. This error occurs in the following code:

class = 'algebra'

The keywords of Python3 are: 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) Use the increment operator when defining a new variable (resulting in "NameError: name 'foobar' is not defined"). Do not use 0 or an empty string as the initial value when declaring a variable. Use the increment operator instead. The sentence spam += 1 is equivalent to spam = spam + 1, which means that spam needs to specify a valid initial value. This error occurs in the following code:

spam = 0 
spam += 42 
eggs += 42

14) Using a local variable in a function before defining the local variable (at this time there is a global variable with the same name as the local variable) (resulting in "UnboundLocalError: local variable 'foobar ' referenced before assignment") It is very complicated to use a local variable in a function when there is a global variable with the same name. The rule of thumb is: if anything is defined in the function, if it is only used in the function, then it is Local, otherwise it is a global variable. This means that you cannot use it as a global variable in a function before defining it. The error occurs in the following code:

someVar = 42 
def myFunction(): 
   print(someVar) 
   someVar = 100 
   myFunction()

15) Trying to use range() to create a list of integers (resulting in "TypeError: 'range' object does not support item assignment") Sometimes you want to get an ordered list 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 the 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 in Range() in Python 2 returns a list value, but in Python 3 the above error will occur)

16) The error lies in the ++ or -- increment and decrement operators. (Resulting in "SyntaxError: invalid syntax") If you are used to other languages ​​such as C++, Java, PHP, etc., you may want to try using ++ or -- to increment and decrement a variable. There is no such operator in Python. The error occurs in the following code:

spam = 1
spam++

Maybe this is what you want to do:

spam = 1 
spam += 1

17）忘记为方法的第一个参数添加self参数（导致“TypeError: myMethod() takes no arguments (1 given)”） 该错误发生在如下代码中：

class Foo(): def myMethod(): 
       print('Hello!') a = Foo() a.myMethod()

 

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