Das deutsche Python-Forum

Code: Alles auswählen

help("for")

Code: Alles auswählen

Sorry, topic and keyword documentation is not available because the Python
HTML documentation files could not be found.  If you have installed them,
please set the environment variable PYTHONDOCS to indicate their location.

Code: Alles auswählen

1 2 3 4 ...
2 4 6 8 ...
3 6 9 12 ...
4 8 12 16 ...
...

Code: Alles auswählen

  ------------------------------------------------------------------------
  
  7.3 The for statement
  
  The for statement is used to iterate over the elements of a sequence
  (such as a string, tuple or list) or other iterable object:

        for_stmt         ::=     "for" target_list[1] "in" expression_list[2] ":" suite[3]
                        ["else" ":" suite[4]]
  
  Download entire grammar as text.[5]
  
  The expression list is evaluated once; it should yield an iterable
  object. An iterator is created for the result of the expression_list.
  The suite is then executed once for each item provided by the iterator,
  in the order of ascending indices. Each item in turn is assigned to the
  target list using the standard rules for assignments, and then the suite
  is executed. When the items are exhausted (which is immediately when the
  sequence is empty), the suite in the else clause, if present, is
  executed, and the loop terminates.
  
  A break statement executed in the first suite terminates the loop
  without executing the else clause's suite. A continue statement executed
  in the first suite skips the rest of the suite and continues with the
  next item, or with the else clause if there was no next item.
  
  The suite may assign to the variable(s) in the target list; this does
  not affect the next item assigned to it.
  
  The target list is not deleted when the loop is finished, but if the
  sequence is empty, it will not have been assigned to at all by the loop.
  Hint: the built-in function range() returns a sequence of integers
  suitable to emulate the effect of Pascal's for i := a to b do; e.g.,
  range(3) returns the list [0, 1, 2].
  
  Warning: There is a subtlety when the sequence is being modified by the
  loop (this can only occur for mutable sequences, i.e. lists). An
  internal counter is used to keep track of which item is used next, and
  this is incremented on each iteration. When this counter has reached the
  length of the sequence the loop terminates. This means that if the suite
  deletes the current (or a previous) item from the sequence, the next
  item will be skipped (since it gets the index of the current item which
  has already been treated). Likewise, if the suite inserts an item in the
  sequence before the current item, the current item will be treated again
  the next time through the loop. This can lead to nasty bugs that can be
  avoided by making a temporary copy using a slice of the whole sequence,
  e.g.,
  
  
  for x in a[:]:
      if x < 0: a.remove(x)
  
  
  ------------------------------------------------------------------------
  Release 2.4.3, documentation updated on 29 March 2006.

Code: Alles auswählen

>>> help('for')

Sorry, topic and keyword documentation is not available because the Python
HTML documentation files could not be found.  If you have installed them,
please set the environment variable PYTHONDOCS to indicate their location.