Can variable names in Python start with an integer?

Question

This is somewhat academic, but nevertheless.

Python syntax forbids starting a variable name with a number, but this can be sidestepped like so:

>>> globals()['1a'] = 1
>>> globals()['1a']
1

Likewise for locals().

Does that mean that Python actually allows it, and that it's just not very visible?

Edit:

My question is not whether it is allowed; I am aware that it is formally not allowed in Python. The question is why can I work around it by addressing globals() directly, and if that breaks certain rules or guidelines, or if it has a good reason/application to allow that.

The parser disallows it, but you can do it by hijacking globals(). But you're not really intended to do that. — Kevin
– Kevin, Commented Jan 31, 2017 at 16:22
You can't use it as a variable later... so it isn't really a variable, even though it is in the same namespace. — tdelaney
– tdelaney, Commented Jan 31, 2017 at 16:26
Your answer (duplicate): stackoverflow.com/questions/342152/… — MichaelMMeskhi
– MichaelMMeskhi, Commented Jan 31, 2017 at 16:37
@steffan Modifying locals doesn't work [docs.python.org/3/library/functions.html#locals] so you can only do this trick in globals — DavidW
– DavidW, Commented Feb 3, 2017 at 11:58
@DavidW I just tried it... it works in Python 3.6.0 and 2.7.13 — steffen
– steffen, Commented Feb 3, 2017 at 12:51

Uriel · Accepted Answer · 2017-01-31 22:37:17Z

31

Python parser forbids naming variables that way, for the sake of parsing numbers and variables separately, as naming a variable 1e1 would create a chaos - is it the number 10.0 or the variable 1e1?

"Python, please output for me 1e1!" - "Why is it 10.0? I stored 100 over there!"

But the variables are actually stored in a way that allows binding a string that starts with a number to a value, because that feature is no harm in hashing maps of any kind, and so using this "trick" you can achieve your wanted numeral-prefixed-name variable without hurting the parser severability.

I would say that technically, naming variables in that manner is not a violation to python guidelines, but it is highly discouraged, and as a rule unnecessary. Using globals for injecting variables is known as a very bad practice and this case should not be an outstanding.

Of course, python could have used an encloser to numerals like strings, say *123*, but I believe the intent of inventing python was to make programming easier, not stretching the limits of variable naming space.

Practically speaking, if you must use number-headed names you better do it with your own dictionary, rather than globals:

>>> number_headed_vars = {'1a': 100}
>>> number_headed_vars['1a']
100

That way you can create your own variables system - and avoid abusing globals().

edited Jan 31, 2017 at 22:37

answered Jan 31, 2017 at 16:32

Uriel

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Minion Jim Over a year ago

I suspect this was not implemented more because it would cause ambiguity in the grammar of the old parser (<=3.8). I believe it could be implemented with the new PEG parser (>=3.9) but I doubt the core-devs would deem it a necessary addition and therefore would vote against any PEP proposing this.

Ofer Arial · Accepted Answer · 2017-01-31 16:31:51Z

8

This is what you can and can't do with that 1a in globals. You can't really use it in a variable, unless you use all of it's definition in globals (I mean accessing that dictionary), which makes it very uncomfortable for usage (another reason for not doing that).

Basically, 1a is not a real variable as a1 , as shown in the following output:

>>> globals()['1a'] = 1
>>> globals()['1a']
1

>>> a = 1a
File "<stdin>", line 1
    a = 1a
         ^
SyntaxError: invalid syntax

>>> a = globals()['1a']
>>> a
1

>>> globals()['a1'] = 5
>>> a = a1
>>> a
5

answered Jan 31, 2017 at 16:31

Ofer Arial

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1 Comment

Uriel Over a year ago

Hi, came back to this question because of upvotes wave, and seen the sentence "1a is not a real variable.". While this makes sense, it isn't true. The problem isn't that 1a isn't a "real" variable. The reason you can't use it in your code is because the parser can not tokenize 1a as a variable (if it could, it would have worked perfectly, since is it stored normally in the globals). The rest is all good.

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