Hi, In that message there was a logical step that I don't follow:
For example: '{a.foo}'.format(a=b[c])
If we limit f-strings to just what str.format() string expressions can represent, it would be impossible to represent this with an f-string, without an intermediate assignment.
For example: f'{a[2:3]:20d}'
We need to extract the expression "a[2:3]" and the format spec "20d". I can't just scan for a colon any more, I've got to actually parse the expression until I find a "}", ":", or "!" that's not part of the expression so that I know where it ends.
There was a solution to this that came up early in the discussion, moving the identifier only: f'{x}{y}' --> '{}{}'.format(x, y) f'{x:10}{y[name]}' --> '{:10}{[name]}'.format(x, y) I missed the part where this was rejected. As far as I can tell from your message, it is because it would be hard to parse? But, it seems no harder than other solutions. I've whipped up a simple implementation below. Also, Guido sounds supportive of your general process, but to my knowledge has not explicitly called for arbitrary expressions to be included. Perhaps he could do that, or encourage us to find a more conservative solution? Sorry to be a pain, but I think this part is important to get right. -Mike Simple script to illustrate (just ascii, only one format op supported). TL;DR: the idea is to grab the identifier portion by examining the class of each character, then move it over to a .format function call argument. import string idchars = string.ascii_letters + string.digits + '_' # + unicode letters capture = None isid = None fstring = '{a[2:3]:20d}' #~ fstring = '{a.foo}' identifier = [] fmt_spec = [] for char in fstring: print(char + ', ', end='') if char == '{': print('start_capture ', end='') capture = True isid = True elif char == '}': print('end_capture') capture = False break else: if capture: if (char in idchars) and isid: identifier.append(char) else: isid = False fmt_spec.append(char) identifier = ''.join(identifier) fmt_spec = ''.join(fmt_spec) print() print('identifier:', repr(identifier)) print('fmt_spec: ', repr(fmt_spec)) print('result: ', "'{%s}'.format(%s)" % (fmt_spec, identifier)) And the results: ＞python3 fstr.py {, start_capture a, [, 2, :, 3, ], :, 2, 0, d, }, end_capture identifier: 'a' fmt_spec: '[2:3]:20d' result: '{[2:3]:20d}'.format(a) On 08/04/2015 11:32 AM, Eric V. Smith wrote:
Actually, a better link is: https://mail.python.org/pipermail/python-ideas/2015-July/034729.html where I discuss the pros and cons of str.format-like expressions, versus full expressions. Plus, Guido's response.