## Converting from SLiM time to tskit time¶

Note

This is a nitpicky, document-the-details section. Hopefully, you don’t have to deal with the specifics of converting between tskit and SLiM time, but this page is here for you if you do.

SLiM is a forwards simulator, while the tree sequence format thinks about things retrospectively, and so works with times in units of time ago. Mostly, you don’t have to convert between the two, unless you want to match up information in a tree sequence with information written out by SLiM itself. In other words, SLiM’s time counter measures the number of time steps (“generations”) since the start of the simulation, and times in the tree sequence record how long before the end of the simulation. However, there are Some Details, and off-by-one errors are easy to make, so we’ll spell it out in detail.

SLiM’s time counter is called the “generation” (although a “year” or “life cycle” would be a more appropriate name for a nonWF model). The SLiM generation starts at 1, and records which round of the life cycle the simulation is in. However, the order of the life cycle differs between WF and nonWF models: in a WF model, it is “early $$\to$$ birth $$\to$$ late”, while in a nonWF model, it is “birth $$\to$$ early $$\to$$ late”. Usually, the first set of individuals are created in the early() phase of generation 1, and so in a WF model reproduce immediately, in the same generation they were “born”. Parents and offspring cannot have the same birth time in the tree sequence, and so some clever bookkeeping was required. You’ll want to refer to the tables below to see what’s going on. “Time” in a tree sequence is actually time ago, or time before the tree sequence was recorded. To obtain this number, and ensure that offspring cannot have the same birth time-ago in the tree sequence as their parents, SLiM also keeps track of “how many birth phases of the life cycle have happened so far” (the column “# births” in the tables). As the simulation goes along, tskit time ago is recorded as minus one times the number of birth phases so far. When the tree sequence is output, the current cumulative number of birth phases is added to this, so “tskit time ago” is, equivalently, “how many birth phases happened since this time”. In a nonWF model, the two counters (“generation” and “number of birth phases”) are always in sync; but in a WF they are not (during early). The extra wrinkle this introduces is that the correspondence between “tskit time ago” and “SLiM time” depends on which phase the tree sequence was recorded in, but only for WF models.

To help keep all this straight, here are schematics for WF and nonWF models. (To see the nonWF model, click on the tab.)

For a WF model, the SLiM generation (first column) can be obtained by subtracting the tskit time ago from the SLiM generation at time of output only during the same stage that output occured in.

generation

stage

# births

tskit time ago, early output

tskit time ago, late output

1

early

0

$$\leftarrow$$ add subpops

n-1

n

1

birth

1

n-2

n-1

1

late

1

n-2

n-1

2

early

1

n-2

n-1

2

birth

2

n-3

n-2

2

late

2

n-3

n-2

3

early

2

n-3

n-2

3

birth

3

n-4

n-3

3

late

3

n-4

n-3

$$\downarrow$$

$$\cdots$$

$$\downarrow$$

$$\uparrow$$

$$\uparrow$$

n-2

early

n-3

2

2

n-2

birth

n-2

1

2

n-2

late

n-2

1

2

n-1

early

n-2

1

2

n-1

birth

n-1

0

1

n-1

late

n-1

0

1

n

early

n-1

treeSeqOutput $$\to$$

0

1

n

birth

n

0

n

late

n

treeSeqOutput $$\to$$

0

Note that for nonWF models the SLiM generation (first column) can always be obtained by subtracting the tskit time ago from the SLiM generation at time of output.

generation

stage

# births

tskit time ago, early output

tskit time ago, late output

1

birth

1

n-1

n-1

1

early

1

$$\leftarrow$$ add subpops

n-1

n-1

1

late

1

n-1

n-1

2

birth

2

n-2

n-2

2

early

2

n-2

n-2

2

late

2

n-2

n-2

3

birth

3

n-3

n-3

3

early

3

n-3

n-3

3

late

3

n-3

n-3

$$\downarrow$$

$$\cdots$$

$$\downarrow$$

$$\uparrow$$

$$\uparrow$$

n-2

birth

n-2

2

2

n-2

early

n-2

2

2

n-2

late

n-2

2

2

n-1

birth

n-1

1

1

n-1

early

n-1

1

1

n-1

late

n-1

1

1

n

birth

n

0

0

n

early

n

treeSeqOutput $$\to$$

0

0

n

late

n

treeSeqOutput $$\to$$

0

When the tree sequence is written out, SLiM records the value of its current generation, which can be found in the metadata: ts.metadata['SLiM']['generation'] (or, the ts.slim_generation attribute). In most cases, the “SLiM time” referred to by a time in the tree sequence (i.e., the value that would be reported by sim.generation within SLiM at the point in time thus referenced) can be obtained by subtracting time from ts.slim_generation. However, in WF models, birth happens between the “early()” and “late()” stages, so if the tree sequence was written out using sim.treeSeqOutput() during “early()” in a WF model, the tree sequence’s times measure time before the last set of individuals are born, i.e., before SLiM time step ts.slim_generation - 1. The stage that the tree sequence was saved is recorded in the metadata of the tree sequence, as ts.metadata['SLiM']['stage']. Using this, we can convert from the times of a tree sequence ts to SLiM time as follows:

def slim_time(ts, time, stage):
slim_time = ts.slim_generation - time
and stage == "late"):
slim_time -= 1
and stage == "early"):
slim_time += 1
return slim_time


This is what is computed by the SlimTreeSequence.slim_time() method (which also has a stage argument).

Some of the other methods in pyslim – those that depend on SlimTreeSequence.individuals_alive_at() – need you to tell them during which stage the tree sequence was saved with sim.treeSeqOutput, and need this to be the same as the stage that any individuals were saved with sim.treeSeqRememberIndividuals. This argument, remembered_stage, defaults to “late()”; we recommend that you also default to always Remembering individuals, and saving out the tree sequence, during “late()” as well, unless you have good reason not to. (This means you must specify the stage of the block in your SLiM script, since the stage defaults to “early()”!)

The entries of the top-level metadata dict are read-only. So, you might think that tables.metadata["SLiM"]["model_type"] = "nonWF" would switch the model type, but this in fact (silently) does nothing. To modify the top-level metadata, we must (a) work with tables (as tree sequences are immutable, and (b) extract the metadata dict, modify the dict, and copy it back in. Instead, you should do

md = tables.metadata
md["SLiM"]["model_type"] = "nonWF"


Modifying the top-level metadata could be used to set spatial bounds on an annotated msprime simulation, for instance. (This is recorded in the population metadata.)

### Modifying SLiM metadata in tables¶

To modify the metadata that pyslim has introduced into the tree sequence produced by a coalescent simulation, or the metadata in a SLiM-produced tree sequence, what we do is (a) extract the metadata (as a list of dicts), (b) modify them, and then (c) write them back into the tables. For instance, to set the ages of the individuals in the tree sequence to random numbers between 1 and 4, and write out the resulting tree sequence:

tables = ts.tables
ind_md = [ind.metadata for ind in tables.individuals]
for md in ind_md:
md["age"] = random.choice([1,2,3,4])

[ims.validate_and_encode_row(md) for md in ind_md])

# check that it worked:
print("First ten ages:", [mod_ts.individual(i).metadata["age"] for i in range(10)])
for ind in mod_ts.individuals():
assert ind.metadata['age'] in [1, 2, 3, 4]

# save out the tree sequence
mod_ts.dump("modified_ts.trees")

First ten ages: [3, 1, 1, 3, 4, 4, 3, 2, 2, 3]


## Technical details¶

SLiM records additional information in the metadata columns of Population, Individual, Node, and Mutation tables, in a binary format using the python struct module. See tskit’s metadata documentation for details on how this works. Nothing besides this binary information can be stored in the metadata of these tables if the tree sequence is to be used by SLiM, and so when pyslim annotates an existing tree sequence, anything in those columns is overwritten. For more detailed documentation on the contents and format of the metadata, see the SLiM manual.

Of particular note is that nodes and populations may have empty metadata. SLiM will not use the metadata of nodes that are not associated with alive individuals, so this can safely be omitted (and makes recapitation easier). And, populations not used by SLiM will have empty metadata. All remaining metadata are required (besides edges and sites, whose metadata is not used at all).

In previous versions of pyslim, SLiM-specific metadata was provided as customized objects: for instance, for a node n provided by a SlimTreeSequence, we’d have n.metadata as a NodeMetadata object, with attributes n.metadata.slim_id and n.metadata.is_null and n.metadata.genome_type. However, with tskit 0.3, the capacity to deal with structured metadata was implemented in tskit itself, and so pyslim shifted to using the tskit-native metadata tools. As a result, parsed metadata is provided as a dictionary instead of an object, so that now n.metadata would be a dict, with entries n.metadata["slim_id"] and n.metadata["is_null"] and n.metadata["genome_type"]. Annotation should be done with tskit methods (e.g., packset_metadata).

For now, the old-style metadata is still available: passing the argument legacy_metadata=True to load() will produce a tree sequence whose metadata is just as before, and so all previously-written scripts that depend on metadata processing should work, unchanged. Restating this:

Note

To make an script that relied on previous metadata parsing work, it should suffice to add legacy_metadata=True to calls producing SlimTreeSequences, e.g., replacing pyslim.load("file.trees") with pyslim.load("file.trees", legacy_metadata=True), and ts.simplify(nodes) with pyslim.SlimTreeSequence(ts.simplify(nodes), legacy_metadata=True). If this fails, please file an issue on github.

Here are more detailed notes on how to migrate a script from the legacy metadata handling.

1. Use top-level metadata instead of slim_provenance: previously, information about the model type and the time counter (generation) in SLiM was provided in the Provenances table, made available through the ts.slim_provenance object. This is still available but deprecated, and should be obtained from the top-level metadata object, ts.metadata["SLiM"]. So, in your scripts ts.slim_provenance.model_type should be replaced with ts.metadata["SLiM"]["model_type"], and (although it’s not deprecated), probably ts.slim_generation should probably be replaced with ts.metadata["SLiM"]["generation"].

2. Switch metadata objects to dicts: if md is the metadata property of a population, individual, or node, this means replacing md.X with md["X"]. The migration_records property of population metadata is similarly a list of dicts rather than a list of objects, so instead of ts.population(1).metadata.migration_records[0].source_subpop we would write ts.population(1).metadata["migration_records"][0]["source_subpop"].

Mutations were previously a bit different - if mut is a mutation (e.g., mut = ts.mutation(0)) then mut.metadata was previously a list of MutationMetadata objects. Now, mut.metadata is a dict, with a single entry: mut.metadata["mutation_list"] is a list of dicts, each containing the information that was previously in the MutationMetadata objects. So, for instance, instead of mut.metadata[0].selection_coeff we would write mut.metadata["mutation_list"][0]["selection_coeff"].

3. The decode_X and encode_X methods are now deprecated, as this is handled by tskit itself. For instance, encode_node would take a NodeMetadata object and produce the raw bytes necessary to encode it in a Node table, and decode_node would do the inverse operation. This is now handled by the relevant MetadataSchema object: for nodes one can obtain this as nms = ts.tables.nodes.metadata_schema, which has the methods nms.validate_and_encode_row and nms.decode_row. Decoding is for the most part not necessary, since the metadata is automatically decoded, but pyslim.decode_node(raw_md) could be replaced by nms.decode_row(raw_md). Encoding is necessary to modify tables, and pyslim.encode_node(md) can be replaced by nms.validate_and_encode_row(md) (where furthermore md should now be a dict rather than a NodeMetadata object).

4. The annotate_X_metadata methods are deprecated, as again tskit has tools to do this. These methods would set the metadata column of a table - for instance, if metadata is a list of NodeMetadata objects, then annotate_node_metadata(tables, metadata) would modify tables.nodes in place to contain the (encoded) metadata in the list metadata. Now, this would be done as follows (where now metadata is a list of metadata dicts):

metadata = [ {'slim_id': k, 'is_null': False, 'genome_type': 0}
for k in range(tables.nodes.num_rows) ]

If speed is an issue, then encode_row can be substituted for validate_and_encode_row, but at the risk of missing errors in metadata.
5. the extract_X_metadata methods are not necessary, since the metadata in the tables of a TableCollection are automatically decoded. For instance, [ind.metadata["sex"] for ind in tables.individuals] will obtain a list of sexes of the individuals in the IndividualTable.