SyntaxError: keyword can't be an expression while creating a dictionary

dictionary, python

Solution

The problem with `rack.session` is that python thinks that you're trying to use the value of expression `rack.session` and pass it to `dict()`, which is incorrect because `dict()` expects you to pass variables names when you're using keyword arguments, these variables name are then converted to strings when the dict is created.

Simple example:

>>> dict('val' = 'a')     
  File "<ipython-input-21-1cdf9688c191>", line 1
SyntaxError: keyword can't be an expression

So, you can't use an object on the left side of `=`, you can only use a valid identifier.

Byte code makes it even more clear what happens with `rack.session`:

>>> import dis
>>> dis.dis(lambda : dict(rack.session , val))
  1           0 LOAD_GLOBAL              0 (dict)
              3 LOAD_GLOBAL              1 (rack)   # load the object `rack`
              6 LOAD_ATTR                2 (session)# use the value of it's attribute
                                                    # `session`
              9 LOAD_GLOBAL              3 (val)
             12 CALL_FUNCTION            2
             15 RETURN_VALUE

So, with `rack.session = val`, python will think that you're trying to use the value returned from `rack.session` and pass it to `dict`, which is incorrect. Secondly `rack.session` isn't a valid identifier as dots(`.`) are not allowed in python identifiers.

This is applicable to any function in python not even `dict`, a keyword argument must be a valid identifier.

From the docs:

keyword_item   ::=  identifier "=" expression

Valid examples:

>>> dict(foo = 1, bar = '2')
{'foo': 1, 'bar': '2'}

For your example you can simply do:

>>> val = 'CookieVal'
>>> name = 'rack.session'
>>> dict(((name,val),))
{'rack.session': 'CookieVal'}
#or
>>> {name:val}
{'rack.session': 'CookieVal'}

Problem

I got two strings retrieved from a cookie ``` name = 'rack.session' val = 'CookieVal' ``` Using them I would like to build a dictionary ``` cookies = dict(rack.session=val) ``` but `SyntaxError: keyword can't be an expression` So I tried to escape the (.) dot ``` re.escape(name) ``` ... but it raises the same error How is this possible? According to Python `type()` name is a string: ``` type(name) <class 'str'> ``` Why is Python mixing up strings and expressions?

Original source