What does this , operator do when you use it with = sign?

Question

i came across this completely randomly just curious because this this doesn't even look like a list .

Answer 1

This is a weird side-effect of the way the auto-quoting feature is implemented in IPython. In particular, every line entered at the IPython terminal is pattern-matched against this regex pattern:

import re
line_split = re.compile("""
             ^(\s*)               # any leading space
             ([,;/%]|!!?|\?\??)?  # escape character or characters
             \s*(%{0,2}[\w\.\*]*)     # function/method, possibly with leading %
                                  # to correctly treat things like '?%magic'
             (.*?$|$)             # rest of line
             """, re.VERBOSE)

In the case of the input ', = what iss this', this results in the following assignments:

pre, esc, ifun, the_rest = line_split.match(', = what iss this').groups()
print(repr(pre))
# ''

print(repr(esc))
# ','

print(repr(ifun))
# ''

print(repr(the_rest))
# '= what iss this'

Since esc is a comma, the AutoHandler prefilter reaches this if-else statement:

    if esc == ESC_QUOTE:
        # Auto-quote splitting on whitespace
        newcmd = '%s("%s")' % (ifun,'", "'.join(the_rest.split()) )

which modifies the command to become

In [19]: ifun=''

In [20]: the_rest='= what iss this'

In [21]: newcmd = '%s("%s")' % (ifun,'", "'.join(the_rest.split()) )

In [22]: newcmd
Out[22]: '("=", "what", "iss", "this")'

So in summary,

• the initial comma triggers IPython's auto-quoting feature.
• since no valid function name was found, the ifun is an empty string

• auto-quoting quotes the rest of the command string and forms

  '%s("%s")' % (ifun,'", "'.join(the_rest.split()) )
  

  as the new command. This "command" is then evaluated.

Hence, the result returned is the tuple ("=", "what", "iss", "this").

Answer 2

Just using ? in IPython to see Introduction and overview of IPython's features.

2. Auto-Quoting

   You can force auto-quoting of a function's arguments by using ',' as the first character of a line. For example::

     In [1]: ,my_function /home/me   # becomes my_function("/home/me")
   

   If you use ';' instead, the whole argument is quoted as a single string (while ',' splits on whitespace)::

     In [2]: ,my_function a b c   # becomes my_function("a","b","c")
     In [3]: ;my_function a b c   # becomes my_function("a b c")
   

   Note that the ',' MUST be the first character on the line! This won't work::

     In [4]: x = ,my_function /home/me    # syntax error
   

EDIT: Sorry for the = explanation. As @randomir said, the = op discusses is here.