Sorting TKey in python 3

[Wile_E_Coyote]

I am trying to “upgrade” some old ROOT Python 2 scripts to Python 3 (needs to work with ROOT 6.20 and 6.34).

Could you please advise me on how to fix it:

if obj.IsA().InheritsFrom("TDirectory"):
    keys = obj.GetListOfKeys()
        for key in sorted(keys):
            # do something

I now get (ROOT 6.20, Python 3.6.8):

    for key in sorted(keys):
TypeError: '<' not supported between instances of 'TKey' and 'TKey'

I’ve found something that seems to work, but I’m not sure that “keys.Sort()” is the same as “sorted(keys)” and that it is safe (in all possible cases):

if obj.IsA().InheritsFrom("TDirectory"):
    keys = obj.GetListOfKeys()
    keys.Sort()
        for key in keys:
            # do something

[mdessole]

Hello @Wile_E_Coyote,

let me add in the loop @vpadulan

[vpadulan]

Dear @Wile_E_Coyote ,

Thanks for reaching out! I’m trying to understand your case to give the best advice. My first instinct is that you would like to get the list of keys from a TDirectory, then sort them by their name? If so, I believe the easiest way is to use the key argument to the sorted builtin function. Here is a simple example:

import ROOT

with ROOT.TFile("myfile.root", "recreate") as f:
    a = ROOT.TNamed("a_name", "a_title")
    b = ROOT.TNamed("b_name","b_title")
    # Write in reverse alphabetical order
    f.WriteObject(b)
    f.WriteObject(a)

with ROOT.TFile("myfile.root") as f:
    print("Unsorted")
    tkeylist = f.GetListOfKeys()
    for tkey in tkeylist:
        print(tkey.GetName())

    print("Sorted")
    for tkey in sorted(tkeylist, key = lambda tkey: tkey.GetName()):
        print(tkey.GetName())

[Wile_E_Coyote]

Thanks.
Do you know what the original “sorted(keys)” was doing in the old ROOT with Python 2?
I just want to “reproduce” this behavior.

[vpadulan]

Dear @Wile_E_Coyote ,

Unfortunately I have no concrete knowledge, as I would have also thought it to be illegal since there is no operator< defined for TKey (as your first message states). My best guess is that a previous PyROOT implementation was less strict with type checking and it was sorting based on the address of the TKey (?). If you have the Python 2 environment available you could try to double check this hypothesis yourself by printing id(tkey) and check if they correspond to the addresses of the sorted list.

Cheers,
Vincenzo

[Wile_E_Coyote]

It turns out that “sorted(keys)” dies with that particular ROOT 6.20 plus Python 3.6.8 (CentOS 7) setup (that I need to use).

Then, it seems that when it works, “sorted(keys)” returns the “key name” sorted list (in any ROOT version).

#
# Note: it works with Python 2 and 3
#

import ROOT

F = ROOT.TFile.Open("myfile.root", "recreate")

# Create in "random" alphabetical order
e = ROOT.TNamed("e_name", "b_title")
a = ROOT.TNamed("a_name", "f_title")
b = ROOT.TNamed("b_name", "e_title")
f = ROOT.TNamed("f_name", "a_title")
c = ROOT.TNamed("c_name", "d_title")
d = ROOT.TNamed("d_name", "c_title")

# Write in "random" alphabetical order
F.WriteTObject(c)
F.WriteTObject(d)
F.WriteTObject(b)
F.WriteTObject(e)
F.WriteTObject(a)
F.WriteTObject(f)

F.Write()
F.Close()

F = ROOT.TFile.Open("myfile.root", "read")

tkeylist = ROOT.gDirectory.GetListOfKeys()

print("Sorted (default)")
for tkey in sorted(tkeylist):
    print (id(tkey), " -> ", tkey.GetName())

print("Sorted (GetName)")
for tkey in sorted(tkeylist, key = lambda tkey: tkey.GetName()):
    print (id(tkey), " -> ", tkey.GetName())

print("Unsorted")
for tkey in tkeylist:
    print (id(tkey), " -> ", tkey.GetName())

F.Close()