hello all,

i am having a hard time in using bcftools set set some variants in my vcf to missing.

as i understand missing2ref is no longer used and replaced with setGT. having said that, it is not clear how to use it.

i need to set any variant with genotype quality (GQ) < 20 to missing. this will require that i use the plugin +fill-tags at the first stage, but then how do i use the setGT plugin? how is the filter command written?

i am so lost here. thanks in advance
Below is a Python API solution using the `pyvcf` [submodule](https://sbslee-fuc.readthedocs.io/en/latest/api.html?highlight=VcfFrame.from_dict#module-fuc.api.pyvcf) from the `fuc` [package](https://sbslee-fuc.readthedocs.io/en/latest/readme.html) I wrote.

Imagine you have the following data:
```
>>> from fuc import pyvcf
>>> data = {
...     'CHROM': ['chr1', 'chr1', 'chr1', 'chr1'],
...     'POS': [100, 101, 102, 103],
...     'ID': ['.', '.', '.', '.'],
...     'REF': ['G', 'T', 'A', 'A'],
...     'ALT': ['A', 'C', 'T', 'C'],
...     'QUAL': ['.', '.', '.', '.'],
...     'FILTER': ['.', '.', '.', '.'],
...     'INFO': ['.', '.', '.', '.'],
...     'FORMAT': ['GT:GQ:DP', 'GT:GQ:DP', 'GT:GQ:DP', 'GT'],
...     'A': ['0/0:48:3', '0/0:19:3', './.:.:.', '0/1'],
...     'B': ['0/1:3:5', '0/1:20:8', '0/0:29:12', '0/0'],
...     'C': ['0/0:16:3', '1/0:32:8', '0/1:25:9', '0/0'],
... }
>>> vf = pyvcf.VcfFrame.from_dict([], data)
>>> vf.df
  CHROM  POS ID REF ALT QUAL FILTER INFO    FORMAT         A          B         C
0  chr1  100  .   G   A    .      .    .  GT:GQ:DP  0/0:48:3    0/1:3:5  0/0:16:3
1  chr1  101  .   T   C    .      .    .  GT:GQ:DP  0/0:19:3   0/1:20:8  1/0:32:8
2  chr1  102  .   A   T    .      .    .  GT:GQ:DP   ./.:.:.  0/0:29:12  0/1:25:9
3  chr1  103  .   A   C    .      .    .        GT       0/1        0/0       0/0
```

We can then define a custom method (`one_row`) to be applied to each row to convert genotypes calls with GQ < 20 to missing:

```
>>> def one_row(r):
...     format_list = r.FORMAT.split(':')
...     # This method automatically finds the appropriate missing value ('./.', '.', './.:.:.', etc.).
...     missval = pyvcf.row_missval(r)
...     
...     if 'GQ' in format_list:
...         i = format_list.index('GQ')
...     else:
...         # This row doesn't have GQ, return the entire row as is.
...         return r
...                 
...     def one_gt(g):
...         gq = g.split(':')[i]
...         
...         if gq.isnumeric():
...             gq = int(gq)
...         else:
...             # This genotype doesn't have a numeric GQ value (probably has missing value '.'), keep it as is.
...             return g
...         
...         if gq >= 20:
...             # This genotype has GQ >= 20, keep it as is.
...             return g
...         else:
...             return missval
...        
...     r[9:] = r[9:].apply(one_gt)
...     
...     return r
... 
```

We then apply the method:

```
>>> vf.df = vf.df.apply(one_row, axis=1)
>>> vf.df
  CHROM  POS ID REF ALT QUAL FILTER INFO    FORMAT         A          B         C
0  chr1  100  .   G   A    .      .    .  GT:GQ:DP  0/0:48:3    ./.:.:.   ./.:.:.
1  chr1  101  .   T   C    .      .    .  GT:GQ:DP   ./.:.:.   0/1:20:8  1/0:32:8
2  chr1  102  .   A   T    .      .    .  GT:GQ:DP   ./.:.:.  0/0:29:12  0/1:25:9
3  chr1  103  .   A   C    .      .    .        GT       0/1        0/0       0/0
```

Note that you can easily read and write VCF files too:

```
vf = pyvcf.VcfFrame.from_file('in.vcf')
vf.to_file('out.vcf')
```

Let me know in the comment if you have any questions.