Python：在读取行时，`while`循环如何在python中工作？

Question

How does while loop work in python when reading lines?

state=True #can be set to {anyInterger,True,False}
while state:
    #do a task
    #if task done change state to exit loop

so depending on the state variable while loop is executed, and type(state) can be bool,int

but to read lines from file using while as mentioned below

f=open('test.txt','r')
line_data=[]
line=f.readline()
while line:
    line_data.append(line)
    line=f.readline()
f.close()

after reading all lines finally line='' but type('') is str

So how did while loop terminate?

I knew this could be done better using with and for , reason for not using while loop for this purpose would be helpful

Answer 1

There are a few implicit boolean conversions in Python.
1) bool(0) evaluates to False, any other integers evaluate to True
2) bool('') , bool([]) , bool({}) also evaluate to False . This is useful to test whether a container has any items in it (non-empty strings, lists, dicts etc. evaluate to True )

So in your case readline read the empty string, which got implicitly turned to bool and evaluated to False , therefore the loop has terminated

Answer 2

Whenever you use a variable as a condition to an if or a while loop it is evaluated as a boolean value. In Python an empty string is treated as False , therefore the while loop terminates.

reason for not using while loop for this purpose would be helpful

You should ask for a reason to use a while loop for this purpose.

Compare this:

f = open('test.txt','r')
line_data = []
line = f.readline()
while line:
    line_data.append(line)
    line = f.readline()
f.close()

To that:

with open('test.txt','r') as f:
     lines = f.readlines()

Answer 3

f.readline() is a generator object that returns '' at the end of the file. But '' has a falsy value:

>>> f.readline()
''
>>> bool('')
False

Hence, when the while evaluates the line at EOF (End Of File), the value is the same as False .

The following values are considered False (see the docs ):

 None False zero of any numeric type, for example, 0, 0L, 0.0, 0j. any empty sequence, for example, '', (), []. any empty mapping, for example, {}. instances of user-defined classes, if the class defines a __nonzero__() or __len__() method, when that method returns the integer zero or bool value False 

Answer 4

Empty string is treated as False in python. So, it won't enter the loop. FYI the better way of looping over the lines is

line_data = []
for line in f.readlines():
    line_data.append(line)

You can even do like this also.

with open('test.txt', 'r') as f:
    line_data = [line for line in f.readlines()

Answer 5

You can add:

if len(line) < 2:
    break

or alternatively:

if line == "''":
    break