'str' object has no attribute 'len'

问题

Ive got a method that use to work by checking the first three letters/numbers and making sure they are the same before it continues like so

def combineProcess(request):
    carID1 = request.POST['carID1']
    carID2 = request.POST['carID2']
    for x in range (0,3):
        a += carID1.length(x)
        b += carID2.length(x)
    if a.equals(b):
        //do something

before it use to work now it stopped and i get this error.

Exception Type: UnboundLocalError
Exception Value:    

local variable 'a' referenced before assignment

which i never use to get a few weeks ago didnt change anything so i made a and b global.

def combineProcess(request):
    carID1 = request.POST['carID1']
    carID2 = request.POST['carID2']
    global a,b
    for x in range (0,3):
        a += carID1.length(x)
        b += carID2.length(x)
    if a.equals(b):
        //do something

Now I am getting this error.

Exception Type: NameError
Exception Value:    

name 'a' is not defined

Then i removed the global line and just put this

a = "P"

and got the following error

str object has no attribute length() or len()

which now has me puzzled how has this code stop working and why cant it recognize that a string object has a len() method. mainly I am lost how my code went from working to not working over a two weeks off.

回答1:

Python String len() Method

Syntax

Following is the syntax for len() method −

len( str )

Example

str = "this is string example....wow!!!";
print("Length of the string: ", len(str))

When we run above program, it produces following result −

Length of the string:  32

Reference: Python String len() Method

UnboundLocalError: local variable 'a' referenced before assignment

Explanation

This is because, even though a and b exists, you're also using an assignment statement on the name a and b inside of the function combineProcess(). Naturally, this creates a variable inside the function's scope called a and b.

The Python interpreter sees this at module load time and decides that the global scope's of a and b should not be used inside the local scope, which leads to a problem when you try to reference the variable before it is locally assigned.

Example

Var1 = 1
Var2 = 0
def function():
    if Var2 == 0 and Var1 > 0:
        print("Result One")
    Var1 =- 1

function()

If you run this program, it gives the following error.

UnboundLocalError: local variable 'Var1' referenced before assignment

Since, the value of Var1 is modified, this creates a variable inside the function's scope called Var1. As a result error is reported because of the condition check, Var1 > 0 before the Var1 =- 1 statement.

But if we modify the code as follows.

Var1 = 1
Var2 = 0
def function():
    global Var1
    if Var2 == 0 and Var1 > 0:
        print("Result One")
    Var1 =- 1

function()

Then it works fine.

Note that, if we move the statement Var1 =- 1 before the if condition check, then it will not report any error even if you don't use global Var1 statement. So, the following code works fine.

Var1 = 1
Var2 = 0
def function():
    Var1 =- 1
    if Var2 == 0 and Var1 > 0:
        print("Result One")

function()

Reference: See this SO answer.

NameError: name 'a' is not defined

Explanation

Probably you are getting this error because in Python, you cannot compare two strings using equals() method. There is no such method exists.

Example: Comparing two strings

You can use > , < , <= , <= , == , != to compare two strings. Python compares string lexicographically i.e using ASCII value of the characters. For example, to compare two strings for equality, you can do as follows.

if string1 == string2:
    // do something

回答2:

I assume you've come from a language such as Java where you call functions directly on strings. You could do that in Python:

>>> A = "Hello"
>>> B = "Hello"
>>> A.__len__()
5

>>> A.__eq__(B)
True

However, the proper way of doing it is like so:

>>> len(A)
5

>>> A == B
True

With that being said, I'm not sure what you're trying to accomplish with the length function there. You said your code is checking whether the first 3 letters of the 2 strings are the same - you can do that like so:

A = "car123"
B = "car456"

print(A[:3] == B[:3]) # -> True

This uses Python's slice notation to get the first 3 characters of each string, and compares these slices with each other.

来源：https://stackoverflow.com/questions/41368066/str-object-has-no-attribute-len

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python

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loops

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