Using p4

Using the p4 script

As usual for a Python package, you can import p4 from within Python, by import p4. However, my favourite way of using it is with the p4 script (which should have been installed, hopefully somewhere in your path). The p4 script does a from p4 import *, so your top-level namespace becomes instantly cluttered, but then you do not need to precede everything with p4.. The p4 script allows you to read in phylogenetic data and tree files at the command line. To see if it is installed and working, say:

p4 --help

Some peculiarities, bugs, and “features”

  • P4 doesn’t do searches with ML. It only evaluates explicit trees. It will do tree searches with Bayesian analysis.

  • Arrays and such in Nexus files are 1-based, while in Python and p4 they are zero-based. Nexus files are case-insensitive in p4, as they should be; but elsewhere in p4, as in Python, they are case-sensitive.

  • Nexus trees do not require a Taxa or Data block. Counter to the Nexus standard, having different trees with different taxa in the same trees block is acceptable in p4. However, if you do supply a taxa block, p4 will use it to enforce those taxon names in the trees.

  • Often the results are not written automatically: you have to tell p4 to write the result. However, you can tell it how you would like the output formatted. Since the interface is a programming language you can do whatever you want with the numbers.

  • You give starting values for optimization. If the params are already optimized, then it shouldn’t take long for re-optimization.

  • The default branch length in trees is 0.1

  • When reading Nexus files, p4 recognizes datatype DNA, but not RNA. If you give it datatype nucleotide, p4 will assume you mean DNA.

  • The Nexus spec does not allow names that are all numerals, and by default p4 follows that rule. You can (eg temporarily) override that behaviour and allow all-digit names by True-ing the variable var.nexus_allowAllDigitNames:

    var.nexus_allowAllDigitNames = True
  • The symbols -, ?, and . are the only ones allowed for gaps, unknown, and matchchars, respectively.

The var object, the global bucket


When you import p4, or start the p4 script, you import var, which is an object that holds things like lists and flags that are generally useful to you and p4.

With the p4 script you can read things from the command line, via:

p4 aFile anotherFile

If those files contain phylogenetic data, they get put in

  • var.sequenceLists (a list)
  • var.alignments (a list)
  • var.trees (a list)
  • var.nexusSets (not a list; if it exists it is a NexusSets object)

The read() function

The usual way to get phylogenetic data and trees into p4 is with the read() function. This function is used in scripts that you might write; it is also used within the p4 script to read in files from the command line.

Nexus files or files containing trees often contain multiple things which would be turned into multiple Python objects, so a command like:

myAlignment = readNexus('myData.nex') # Doesn't work

would not always work. The read() function does not return anything; rather, when you read in alignments from files, Alignment objects are made and stuffed into var.alignments, a list, as explained above in The var object, the global bucket. So a typical script might start out by reading in data and trees, like this:

a = var.alignments[0]
t = var.trees[0]

There might be several trees in myTreeFile.nex, and when the file is read in they are all converted to Tree objects and put in the var.trees list. So if you want one of those trees, you have to say which one. To get the first one, say something like:

firstTree = var.trees[0]

When you read in files from the command line using the p4 script, you can use file name globbing, ie use wildcards, as:

p4 *.phy

In that case the shell does the file name expansion. You can also do the same sort of thing with read(), as in:


If you want to make p4 forget the trees that it has read, say:

var.trees = []

You can do the same for alignments and sequenceLists.

If you are sure that the file that you are trying to read has only one thing (Tree, Alignment, SequenceList), then you can use p4.func.readAndPop(). This is handy if for example you are reading in a bunch of sequence list files and one of them happens to have all its sequences the same length – so it gets promoted to an Alignment object and gets put in var.alignments. If you use p4.func.readAndPop() then you would not need to check.

# awkward using read()
for fName in myFileList:
    var.alignments = []
    var.sequenceLists = []
        sl = var.sequenceLists[0]
    except IndexError:
        sl = var.alignments[0]

# easier using readAndPop()
for fName in myFileList:
    sl = func.readAndPop(fName)   # SequenceList or Alignment

The dump() function and methods

There is a function, p4.func.dump() that gives a quick summary of files that you have read, and objects that have been made and placed in var.trees, var.alignments, and so on. It does not know about alignments and such that are not in var.*.

Several classes have dump() methods as well. For example, to see inside trees in fine detail, you can use the p4.tree.Tree.dump() method, for example:




To see details about models, use Model.dump(), for example: