Python built-in function iter

Python built-in functions iter English document:

Iter (object [, sentinel])

Return an iterator object. the first argument is interpreted very differently depending on the presence of the second argument. without a second argument, object must be a collection object which supports the iteration protocol (the _ iter _ () method ), or it must support the sequence protocol (the _ getitem _ () method with integer arguments starting at 0 ). if it does not support either of those protocols, TypeError is raised. if the second argument, sentinel, is given, then object must be a callable object. the iterator created in this case will call object with no arguments for each call to its _ next _ () method; if the value returned is equal to sentinel, StopIteration will be raised, otherwise the value will be returned.

One useful application of the second form of iter () is to read lines of a file until a certain line is reached. the following example reads a file until the readline () method returns an empty string:

With open('mydata.txt ') as fp: for line in iter (fp. readline ,''):
Process_line (line)

Note:

1. the function returns an iteratable object.

2. when the second parameter is not provided, the first parameter must be an iterative protocol (that is, the _ iter _ () method is implemented) (Dictionary, set, and immutable set), or support the sequence protocol (that is, the _ getitem _ () method is implemented to receive an integer parameter starting from 0). Otherwise, an error is returned.

>>> A = iter ({'a': 1, 'B': 2}) # Dictionary set >>>
 
  
>>> Next (a) 'A' >>> next (A) 'B' >>> next (a) Traceback (most recent call last): File"
  
   
", Line 1, in
   
    
Next (a) StopIteration >>> a = iter ('ABC') # string sequence >>>
    
     
>>> Next (a) 'A' >>> next (a) 'B' >>> next (a) 'C' >>>> next () 'D '> next (a) Traceback (most recent call last): File"
     
      
", Line 1, in
      
        Next (a) StopIteration
      
     
    
   
  
 

3. when the second parameter sentinel is provided, the first parameter must be a Callable object. This Callable object is called when the _ next _ method is called. when the returned value is the same as the sentinel value, a StopIteration exception is thrown to terminate the iteration.

# Define class >>> class IterTest: def _ init _ (self): self. start = 0 self. end = 10 def get_next_value (self): current = self. start if current <self. end: self. start + = 1 else: raise StopIteration return current >>> iterTest = IterTest () # instantiate class >>> a = iter (iterTest. get_next_value, 4) # iterTest. get_next_value is a Callable object, and the sentinel value is 4 >>>
 
  
>>> Next (a) >>>> next (a) >>> next () # iteration to 4 terminate Traceback (most recent call last): File"
  
   
", Line 1, in
   
    
Next (a) StopIteration