python/Pyqt5 - how to avoid eval while using ast and getting ValueError: malformed string in attemt to improve code safety

Question

I'm trying to prevent to use eval based on an example how-to-avoid-eval-in-python-for-string-conversion using ast. The challange is that there are a dozen of these self.ch%s_label's to be made but the variable for it changes based on user input in the GUI.

My code:

import ast ...etc.

....

channel_no += 1

ch_width  = eval('self.ch%s_label.frameGeometry().width()' %  (channel_no))

When I change it into:

ch_width  = ast.literal_eval('self.ch%s_label.frameGeometry().width()' %  (channel_no))

I'll get the error:

File "c:\python\anac2\lib\ast.py", line 80, in literal_eval return _convert(node_or_string) File "c:\python\anac2\lib\ast.py", line 79, in _convert raise ValueError('malformed string') ValueError: malformed string

Changing the code (using closing " ") retains the error:

ch_width  = ast.literal_eval("'self.ch%s_label.frameGeometry().width()' %  (channel_no)")

What other options are there... Any suggestions?

snakecharmerb · Accepted Answer · 2017-11-25 13:22:52Z

1

You could use getattr to get the attribute from the instance using the dynamically constructed attribute name:

ch_width = getattr(self, 'ch%s_label' % channel_no).frameGeometry().width() Or step by step:

channel_no = 5
attr_name = 'ch%s_label' % channel_no
attr = getattr(self, attr_name)
ch_width = attr.frameGeometry().width()

Using getattr in this way also means you get an AttributeError if an object doesn't have the attribute, as you'd expect.

answered Nov 25, 2017 at 13:22

snakecharmerb

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1 Comment

ZF007 Over a year ago

@ snakecharmerb.. thanks for the clear example of using getattr. I was deeping functools partial but this is much more pythonic using default built-in code. Cheers. ... and it works off-course :-)

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