2to3 Parse Error on Python 3 print() with arguments

[batterystaple456]

BPO	35260
Nosy	@mdickinson

^{Note: these values reflect the state of the issue at the time it was migrated and might not reflect the current state.}

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GitHub fields:

assignee = None
closed_at = <Date 2018-11-16.13:04:44.221>
created_at = <Date 2018-11-15.21:09:43.442>
labels = ['3.7', 'invalid', 'type-bug', 'expert-2to3']
title = '2to3 Parse Error on Python 3 print() with arguments'
updated_at = <Date 2018-11-16.13:04:44.220>
user = 'https://bugs.python.org/batterystaple456'

bugs.python.org fields:

activity = <Date 2018-11-16.13:04:44.220>
actor = 'mark.dickinson'
assignee = 'none'
closed = True
closed_date = <Date 2018-11-16.13:04:44.221>
closer = 'mark.dickinson'
components = ['2to3 (2.x to 3.x conversion tool)']
creation = <Date 2018-11-15.21:09:43.442>
creator = 'batterystaple456'
dependencies = []
files = []
hgrepos = []
issue_num = 35260
keywords = []
message_count = 5.0
messages = ['329966', '329986', '329988', '329992', '329993']
nosy_count = 2.0
nosy_names = ['mark.dickinson', 'batterystaple456']
pr_nums = []
priority = 'normal'
resolution = 'not a bug'
stage = 'resolved'
status = 'closed'
superseder = None
type = 'behavior'
url = 'https://bugs.python.org/issue35260'
versions = ['Python 3.7']

[batterystaple456]

File a.py:

------------------------------------

print("foo", end='bar') # ParseError

---

I think the error is caused by arguments in the Python 3 print() function (e.g. end, sep) that are not ignored by 2to3, and (possibly) treated like Python 2 code.

Console stuff:

---------------------------------------------------

$ pip install 2to3
Requirement already satisfied: 2to3 in .\python37\lib\site-packages (1.0)

$ 2to3 .\a.py
...
RefactoringTool: Can't parse .\a.py: ParseError: bad input: type=22, value='=', context=('', (1, 16))
...

---

[mdickinson]

I'm not sure I understand the report: 2to3 expects a valid Python 2 file as input, and the file a.py isn't a valid Python 2 file: the print line is a SyntaxError in the absence of a from __future__ import print_function.

So yes, this *is* being treated like Python 2 code, but that's what 2to3 is designed to do.

Can you say what you expected to happen here?

[batterystaple456]

I was expecting that like other Python 3 code, since it is already in Python 3, it would not parse it like Python 2.

[mdickinson]

I'm still confused: the main purpose of 2to3 is to take Python 2 code and turn it into Python 3 code. The docs[1] are fairly unambiguous here:

2to3 is a Python program that reads Python 2.x source code and applies a series of fixers to transform it into valid Python 3.x code.

So expecting 2to3 *not* to parse code as though it's Python 2 code seem odd.

[1] https://docs.python.org/3.7/library/2to3.html

[mdickinson]

Closing. 2to3 is working as designed here.