python循环仅运行一次以进行临时测试

Question

I have a python for loop that iterates over a dictionary. The dictionary is very large. For debugging I want to modify the for loop to run only once. How can I limit the for loop to exit after running once.

for key in dic:
  do_some_stuff()
  #after the first iteration exit

In java I can modify the for loop in this way:

for (int i = 0; i < n; i++)
  doSomeStuff();

will be limited like this:

for (int i = 0; i < n, i < 1; i++)
  doSomeStuff();

Answer 1

Just use break :

It terminates the nearest enclosing loop, skipping the optional else clause if the loop has one.

for key in dic:
    do_some_stuff()
    break

If you want to run the loop for several times (more than once), you can use a counter:

for i, key in enumerate(dic):
    do_some_stuff()
    if i > 10:
        break

This will run the loop for 11 times before break ing as index (i) begins from 0 and goes till 10 .

Answer 2

Since you are doing debugging, I would like to recommend you the pdb package: With it, you got much more control over the execution process.

   import pdb
   pdb.set_trace()
   for key in dic:
        do_some_stuff()

Answer 3

You can always slice the dictionary iterator using

from itertools import islice

for k in islice(dic, 1):
    do_something(k)

Then, to disable it, simply change the 1 to None . Thus you could have a debug flag that decides for you:

please_debug = True
for k in islice(dic, 1 if please_debug else None):
    do_something(k)