Is the single underscore “_” a built-in variable in Python?

by

In the standard Python REPL, _ represents the last returned value — at the point where you called len(_), _ was the value 'abc'.

For example:

>>> 10
10
>>> _
10
>>> _ + 5
15
>>> _ + 5
20

This is handled by sys.displayhook, and the _ variable goes in the builtins namespace with things like int and sum, which is why you couldn’t find it in globals().

Note that there is no such functionality within Python scripts. In a script, _ has no special meaning and will not be automatically set to the value produced by the previous statement.

Also, beware of reassigning _ in the REPL if you want to use it like above!

>>> _ = "underscore"
>>> 10
10
>>> _ + 5

Traceback (most recent call last):
  File "<pyshell#6>", line 1, in <module>
    _ + 5
TypeError: cannot concatenate 'str' and 'int' objects

This creates a global variable that hides the _ variable in the built-ins. To undo the assignment (and remove the _ from globals), you’ll have to:

>>> del _

then the functionality will be back to normal (the builtins._ will be visible again).

More Related Contents:

  1. Converting string into datetime
  2. Check if a given key already exists in a dictionary
  3. How to fix: “UnicodeDecodeError: ‘ascii’ codec can’t decode byte”
  4. Python nonlocal statement
  5. Count the frequency that a value occurs in a dataframe column
  6. Plotting in a non-blocking way with Matplotlib
  7. Deleting multiple elements from a list
  8. Check if a file is not open nor being used by another process
  9. Why do some built-in Python functions only have pass?
  10. TypeError: module.__init__() takes at most 2 arguments (3 given)
  11. Get business days between start and end date using pandas
  12. Find index of last occurrence of a substring in a string
  13. Python: Why does (“hello” is “hello”) evaluate as True? [duplicate]
  14. What is the purpose of python’s inner classes?
  15. Python Slice Assignment Memory Usage
  16. Access django models inside of Scrapy
  17. How do I re.search or re.match on a whole file without reading it all into memory?
  18. Python Opencv SolvePnP yields wrong translation vector
  19. How can I parse free-text time intervals in Python, ranging from years to seconds?
  20. How to convert string to byte array in Python
  21. Max in a sliding window in NumPy array
  22. Selenium – Click at certain position
  23. Build a wheel/egg and all dependencies for a python project
  24. python dask DataFrame, support for (trivially parallelizable) row apply?
  25. Python How to use ExcelWriter to write into an existing worksheet
  26. How to retain leading zeros of int variables?
  27. Find non-common elements in lists
  28. Correct Use Of Global Variables In Python 3
  29. Do Python strings end in a terminating NULL?
  30. Can Python’s set absence of ordering be considered random order?

Leave a Comment