BUG: Adding empty dataframes should result in empty blocks #10181

[rekcahpassyla]

Fixes #10181

I added a couple of lines to the dataframe test test_operators to assert that adding two empty dataframes returns a dataframe that is equal (both using assert_frame_equal as well as .equals) to an empty dataframe.

I couldn't get vbench to install (I am running on windows) - the error was:

c:\dev\code\opensource\pandas-rekcahpassyla>pip install git+https://github.com/pydata/vbench
You are using pip version 6.0.8, however version 6.1.1 is available.
You should consider upgrading via the 'pip install --upgrade pip' command.
Collecting git+https://github.com/pydata/vbench
  Cloning https://github.com/pydata/vbench to c:\users\redacted\appdata\local\temp\pip-ruiahg-build
fatal: unable to connect to github.com:
github.com[0: 192.30.252.129]: errno=No error

Clone of 'git://github.com/yarikoptic/vbenchtest.git' into submodule path 'vbench/tests/vbenchtest/vbenchtest' failed
  Complete output from command C:\dev\bin\git\cmd\git.exe submodule update --init --recursive -q:

  ----------------------------------------
  Command "C:\dev\bin\git\cmd\git.exe submodule update --init --recursive -q" failed with error code 1 in c:\users\redacted\appdata\local\temp\pip-ruiahg-build

The test suite failed for me in master with 1 failure. The same failure, but no others, were observed when running in my branch.

[jreback]

pls add the issue number as a comment.

I would also add a couple of other test, things like

df = DataFrame(columns=['A'])
self.assertTrue((df+df).equals(df)

• add for an index, but no columns
• same as above, but specify a dtype='int64'

[rekcahpassyla]

Added more tests and comment as suggested

[jreback]

@rekcahpassyla ok this looks reasonable. pls add a release note to whatsnew/v0.17.0 (you can put in other API changes section).

pls squash as well.

[rekcahpassyla]

Added release note and squashed.

[rekcahpassyla]

I note the build failure, will have a look.

[rekcahpassyla]

I have verified that my latest commit passes the pandas.stats tests (the cause of the build failure). However I haven't squashed it, as I don't know if it would be considered a separate bug (searching the issues list didn't reveal anything). If it is a separate bug, I'm happy to open another issue and cherrypick this commit into another PR.

[rekcahpassyla]

Raised #10195 and proposed #10196 to fix the issue causing https://travis-ci.org/pydata/pandas/builds/63544612 to fail.

[jreback]

pls rebase this (and move the release note to 0.16.2)

[jreback]

I think these are some old travis failures. can you rebase on master. ping when green.

[rekcahpassyla]

@jreback - It is green

[jreback]

thanks!